diff --git a/.github/pull_request_template.md b/.github/pull_request_template.md
index dec0b9fac..60b0b9d8c 100644
--- a/.github/pull_request_template.md
+++ b/.github/pull_request_template.md
@@ -1,11 +1,7 @@
 ## Ticket
 
+<!-- PR title format: `#issue_number: Descriptive name ideally matching ticket name - [sandbox]`-->
 Resolves #00
-<!--Reminder, when a code change is made that is user facing, beyond content updates, then the following are required:
-- a developer approves the PR
-- a designer approves the PR or checks off all relevant items in this checklist.
-
-All other changes require just a single approving review.-->
 
 ## Changes
 
@@ -45,82 +41,63 @@ All other changes require just a single approving review.-->
 
 - [ ] Met the acceptance criteria, or will meet them in a subsequent PR
 - [ ] Created/modified automated tests
-- [ ] Added at least 2 developers as PR reviewers (only 1 will need to approve)
-- [ ] Messaged on Slack or in standup to notify the team that a PR is ready for review
-- [ ] Changes to “how we do things” are documented in READMEs and or onboarding guide
-- [ ] If any model was updated to modify/add/delete columns, makemigrations was ran and the associated migrations file has been commited.
+- [ ] Update documentation in READMEs and/or onboarding guide
 
 #### Ensured code standards are met (Original Developer)
-
-- [ ] All new functions and methods are commented using plain language
-- [ ] Did dependency updates in Pipfile also get changed in requirements.txt?
+<!-- Mark "- N/A" and check at the end of each check that is not applicable to your PR -->
+- [ ] If any updated dependencies on Pipfile, also update dependencies in requirements.txt.
 - [ ] Interactions with external systems are wrapped in try/except
 - [ ] Error handling exists for unusual or missing values
 
 #### Validated user-facing changes (if applicable)
 
-- [ ] New pages have been added to .pa11yci file so that they will be tested with our automated accessibility testing
+- [ ] Tag @dotgov-designers in this PR's Reviewers for design review. If code is not user-facing, delete design reviewer checklist
+- [ ] Verify new pages have been added to .pa11yci file so that they will be tested with our automated accessibility testing
 - [ ] Checked keyboard navigability
 - [ ] Tested general usability, landmarks, page header structure, and links with a screen reader (such as Voiceover or ANDI)
-- [ ] Add at least 1 designer as PR reviewer
 
 ### As a code reviewer, I have
 
 #### Reviewed, tested, and left feedback about the changes
 
 - [ ] Pulled this branch locally and tested it
-- [ ] Reviewed this code and left comments
+- [ ] Verified code meets all checks above. Address any checks that are not satisfied
+- [ ] Reviewed this code and left comments. Indicate if comments must be addressed before code is merged
 - [ ] Checked that all code is adequately covered by tests
-- [ ] Made it clear which comments need to be addressed before this work is merged
-- [ ] If any model was updated to modify/add/delete columns, makemigrations was ran and the associated migrations file has been commited.
-
-#### Ensured code standards are met (Code reviewer)
-
-- [ ] All new functions and methods are commented using plain language
-- [ ] Interactions with external systems are wrapped in try/except
-- [ ] Error handling exists for unusual or missing values
-- [ ] (Rarely needed) Did dependency updates in Pipfile also get changed in requirements.txt?
+- [ ] Verify migrations are valid and do not conflict with existing migrations
 
 #### Validated user-facing changes as a developer
+**Note:** Multiple code reviewers can share the checklists above, a second reviewer should not make a duplicate checklist. All checks should be checked before approving, even those labeled N/A. 
 
 - [ ] New pages have been added to .pa11yci file so that they will be tested with our automated accessibility testing
 - [ ] Checked keyboard navigability
 - [ ] Meets all designs and user flows provided by design/product
 - [ ] Tested general usability, landmarks, page header structure, and links with a screen reader (such as Voiceover or ANDI)
-
-- [ ] Tested with multiple browsers, the suggestion is to use ones that the developer didn't (check off which ones were used)
-  - [ ] Chrome
-  - [ ] Microsoft Edge
-  - [ ] FireFox
-  - [ ] Safari
-
 - [ ] (Rarely needed) Tested as both an analyst and applicant user
 
-**Note:** Multiple code reviewers can share the checklists above, a second reviewers should not make a duplicate checklist
-
 ### As a designer reviewer, I have
 
 #### Verified that the changes match the design intention
 
 - [ ] Checked that the design translated visually
-- [ ] Checked behavior
+- [ ] Checked behavior. Comment any found issues or broken flows.
 - [ ] Checked different states (empty, one, some, error)
 - [ ] Checked for landmarks, page heading structure, and links
-- [ ] Tried to break the intended flow
 
 #### Validated user-facing changes as a designer
 
 - [ ] Checked keyboard navigability
 - [ ] Tested general usability, landmarks, page header structure, and links with a screen reader (such as Voiceover or ANDI)
-
 - [ ] Tested with multiple browsers (check off which ones were used)
   - [ ] Chrome
   - [ ] Microsoft Edge
   - [ ] FireFox
   - [ ] Safari
-
 - [ ] (Rarely needed) Tested as both an analyst and applicant user
 
+### References
+- [Code review best practices](../docs/dev-practices/code_review.md)
+
 ## Screenshots
 
 <!-- If this PR makes visible interface changes, an image of the finished interface can help reviewers
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index ab15c660f..cbe366470 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -14,17 +14,6 @@ There are a handful of things we do not commit to the repository:
 For developers, you can auto-deploy your code to your sandbox (if applicable) by naming your branch thusly: jsd/123-feature-description
 Where 'jsd' stands for your initials and sandbox environment name (if you were called John Smith Doe), and 123 matches the ticket number if applicable.
 
-## Approvals
-
-When a code change is made that is not user facing, then the following is required:
-- a developer approves the PR
-
-When a code change is made that is user facing, beyond content updates, then the following are required:
-- a developer approves the PR
-- a designer approves the PR or checks off all relevant items in this checklist
-
-Content or document updates require a single person to approve.
-
 ## Project Management
 
 We use [Github Projects](https://docs.github.com/en/issues/planning-and-tracking-with-projects/learning-about-projects/about-projects) for project management and tracking.
@@ -39,14 +28,6 @@ Every issue in this respository and on the project board should be appropriately
 
 We also have labels for each discipline and for research and project management related tasks. While this repository and project board track development work, we try to document all work related to the project here as well. 
 
-## Pull request etiquette
-
-- The submitter is in charge of merging their PRs unless the approver is given explicit permission.
-- Do not commit to another person's branch unless given explicit permission.
-- Keep pull requests as small as possible. This makes them easier to review and track changes.
-- Bias towards approving i.e. "good to merge once X is fixed" rather than blocking until X is fixed, requiring an additional review.
-- Write descriptive pull requests. This is not only something that makes it easier to review, but is a great source of documentation. 
-
 ## Branch Naming
 
-Our branch naming convention is `name/topic-or-feature`, for example: `lmm/add-contributing-doc`. 
+Our branch naming convention is `name/issue_no-description`, for example: `lmm/1234-add-contributing-doc`. 
diff --git a/docs/dev-practices/code_review.md b/docs/dev-practices/code_review.md
new file mode 100644
index 000000000..7b054cad5
--- /dev/null
+++ b/docs/dev-practices/code_review.md
@@ -0,0 +1,31 @@
+## Code Review
+
+Pull requests should be titled in the format of `#issue_number: Descriptive name ideally matching ticket name - [sandbox]`
+Pull requests including a migration should be suffixed with ` - MIGRATION`
+
+After creating a pull request, pull request submitters should:
+- Add at least 2 developers as PR reviewers (only 1 will need to approve).
+- Message on Slack or in standup to notify the team that a PR is ready for review.
+- If any model was updated to modify/add/delete columns, run makemigrations and commit the associated migrations file.
+
+## Pull request approvals
+Code changes on user-facing features (excluding content updates) require approval from at least one developer and one designer.
+All other changes require a single approving review.
+
+The submitter is responsible for merging their PR unless the approver is given explicit permission. Similarly, do not commit to another person's branch unless given explicit permission.
+
+Bias towards approving i.e. "good to merge once X is fixed" rather than blocking until X is fixed, requiring an additional review.
+
+## Pull Requests for User-facing changes
+When making or reviewing user-facing changes, test that your changes work on multiple browsers including Chrome, Microsoft Edge, Firefox, and Safari.
+
+Add new pages to the .pa11yci file so they are included in our automated accessibility testing.
+
+## Other Pull request norms
+- Keep pull requests as small as possible. This makes them easier to review and track changes.
+- Write descriptive pull requests. This is not only something that makes it easier to review, but is a great source of documentation. 
+
+## Coding standards
+
+### Plain language
+All functions and methods should use plain language.
\ No newline at end of file
diff --git a/src/registrar/assets/js/uswds-edited.js b/src/registrar/assets/js/uswds-edited.js
index e73f3b6c0..52dc441fc 100644
--- a/src/registrar/assets/js/uswds-edited.js
+++ b/src/registrar/assets/js/uswds-edited.js
@@ -25,6 +25,8 @@
 /**
  * Edits made for dotgov project:
  *  - tooltip exposed to window to be accessible in other js files
+ *  - tooltip positioning logic updated to allow position:fixed
+ *  - tooltip dynamic content updated to include nested element (for better sizing control)
  *  - modal exposed to window to be accessible in other js files
  *  - fixed bug in createHeaderButton which added newlines to header button tooltips
  */
@@ -5938,6 +5940,22 @@ const showToolTip = (tooltipBody, tooltipTrigger, position) => {
     return offset;
   };
 
+  // ---- DOTGOV EDIT (Added section)
+  // DOTGOV: Tooltip positioning logic updated to allow position:fixed
+  const tooltipStyle = window.getComputedStyle(tooltipBody);
+  const tooltipIsFixedPositioned = tooltipStyle.position === 'fixed';
+  const triggerRect = tooltipTrigger.getBoundingClientRect(); //detect if tooltip is set to "fixed" position
+  const targetLeft = tooltipIsFixedPositioned ? triggerRect.left + triggerRect.width/2 + 'px': `50%`
+  const targetTop = tooltipIsFixedPositioned ? triggerRect.top + triggerRect.height/2 + 'px': `50%`
+  if (tooltipIsFixedPositioned) {
+    /* DOTGOV: Add listener to handle scrolling if tooltip position = 'fixed'
+    (so that the tooltip doesn't appear to stick to the screen) */
+    window.addEventListener('scroll', function() {
+      findBestPosition(tooltipBody)
+    });
+  }
+  // ---- END DOTGOV EDIT
+
   /**
    * Positions tooltip at the top
    * @param {HTMLElement} e - this is the tooltip body
@@ -5949,8 +5967,16 @@ const showToolTip = (tooltipBody, tooltipTrigger, position) => {
     const topMargin = calculateMarginOffset("top", e.offsetHeight, tooltipTrigger);
     const leftMargin = calculateMarginOffset("left", e.offsetWidth, tooltipTrigger);
     setPositionClass("top");
-    e.style.left = `50%`; // center the element
-    e.style.top = `-${TRIANGLE_SIZE}px`; // consider the pseudo element
+
+    // ---- DOTGOV EDIT
+    // e.style.left = `50%`; // center the element
+    // e.style.top = `-${TRIANGLE_SIZE}px`; // consider the pseudo element
+
+    // DOTGOV: updated logic for position:fixed
+    e.style.left = targetLeft; // center the element
+    e.style.top = tooltipIsFixedPositioned ?`${triggerRect.top-TRIANGLE_SIZE}px`:`-${TRIANGLE_SIZE}px`; // consider the pseudo element 
+    // ---- END DOTGOV EDIT
+
     // apply our margins based on the offset
     e.style.margin = `-${topMargin}px 0 0 -${leftMargin / 2}px`;
   };
@@ -5963,7 +5989,17 @@ const showToolTip = (tooltipBody, tooltipTrigger, position) => {
     resetPositionStyles(e);
     const leftMargin = calculateMarginOffset("left", e.offsetWidth, tooltipTrigger);
     setPositionClass("bottom");
-    e.style.left = `50%`;
+
+    // ---- DOTGOV EDIT
+    // e.style.left = `50%`;
+
+    // DOTGOV: updated logic for position:fixed
+    if (tooltipIsFixedPositioned){
+      e.style.top = triggerRect.bottom+'px';
+    }
+    // ---- END DOTGOV EDIT
+
+    e.style.left = targetLeft;
     e.style.margin = `${TRIANGLE_SIZE}px 0 0 -${leftMargin / 2}px`;
   };
 
@@ -5975,8 +6011,16 @@ const showToolTip = (tooltipBody, tooltipTrigger, position) => {
     resetPositionStyles(e);
     const topMargin = calculateMarginOffset("top", e.offsetHeight, tooltipTrigger);
     setPositionClass("right");
-    e.style.top = `50%`;
-    e.style.left = `${tooltipTrigger.offsetLeft + tooltipTrigger.offsetWidth + TRIANGLE_SIZE}px`;
+
+    // ---- DOTGOV EDIT
+    // e.style.top = `50%`;
+    // e.style.left = `${tooltipTrigger.offsetLeft + tooltipTrigger.offsetWidth + TRIANGLE_SIZE}px`;
+
+    // DOTGOV: updated logic for position:fixed
+    e.style.top = targetTop;
+    e.style.left = tooltipIsFixedPositioned ? `${triggerRect.right + TRIANGLE_SIZE}px`:`${tooltipTrigger.offsetLeft + tooltipTrigger.offsetWidth + TRIANGLE_SIZE}px`;
+    // ---- END DOTGOV EDIT
+
     e.style.margin = `-${topMargin / 2}px 0 0 0`;
   };
 
@@ -5991,8 +6035,16 @@ const showToolTip = (tooltipBody, tooltipTrigger, position) => {
     // we have to check for some utility margins
     const leftMargin = calculateMarginOffset("left", tooltipTrigger.offsetLeft > e.offsetWidth ? tooltipTrigger.offsetLeft - e.offsetWidth : e.offsetWidth, tooltipTrigger);
     setPositionClass("left");
-    e.style.top = `50%`;
-    e.style.left = `-${TRIANGLE_SIZE}px`;
+
+    // ---- DOTGOV EDIT
+    // e.style.top = `50%`;
+    // e.style.left = `-${TRIANGLE_SIZE}px`;
+
+    // DOTGOV: updated logic for position:fixed
+    e.style.top = targetTop;
+    e.style.left = tooltipIsFixedPositioned ? `${triggerRect.left-TRIANGLE_SIZE}px` : `-${TRIANGLE_SIZE}px`;
+    // ---- END DOTGOV EDIT
+
     e.style.margin = `-${topMargin / 2}px 0 0 ${tooltipTrigger.offsetLeft > e.offsetWidth ? leftMargin : -leftMargin}px`; // adjust the margin
   };
 
@@ -6017,6 +6069,7 @@ const showToolTip = (tooltipBody, tooltipTrigger, position) => {
       if (i < positions.length) {
         const pos = positions[i];
         pos(element);
+
         if (!isElementInViewport(element)) {
           // eslint-disable-next-line no-param-reassign
           tryPositions(i += 1);
@@ -6128,7 +6181,17 @@ const setUpAttributes = tooltipTrigger => {
   tooltipBody.setAttribute("aria-hidden", "true");
 
   // place the text in the tooltip
-  tooltipBody.textContent = tooltipContent;
+  
+  // -- DOTGOV EDIT
+  // tooltipBody.textContent = tooltipContent;
+
+  // DOTGOV: nest the text element to allow us greater control over width and wrapping behavior
+   tooltipBody.innerHTML = `
+    <span class="usa-tooltip__content">
+      ${tooltipContent}
+    </span>`
+  // -- END DOTGOV EDIT
+
   return {
     tooltipBody,
     position,
diff --git a/src/registrar/assets/sass/_theme/_tooltips.scss b/src/registrar/assets/sass/_theme/_tooltips.scss
index 3ab630dc0..58beb8ae6 100644
--- a/src/registrar/assets/sass/_theme/_tooltips.scss
+++ b/src/registrar/assets/sass/_theme/_tooltips.scss
@@ -28,3 +28,47 @@
 #extended-logo .usa-tooltip__body {
   font-weight: 400 !important;
 }
+
+.domains__table {
+  /*
+  Trick tooltips in the domains table to do 2 things...
+  1 - Shrink itself to a padded viewport window
+  (override width and wrapping properties in key areas to constrain tooltip size)
+  2 - NOT be clipped by the table's scrollable view
+  (Set tooltip position to "fixed", which prevents tooltip from being clipped by parent
+  containers. Fixed-position detection was added to uswds positioning logic to update positioning
+  calculations accordingly.)
+  */
+  .usa-tooltip__body {
+    white-space: inherit;
+    max-width: fit-content; // prevent adjusted widths from being larger than content
+    position: fixed; // prevents clipping by parent containers
+  }
+  /*
+  Override width adustments in this dynamically added class
+  (this is original to the javascript handler as a way to shrink tooltip contents within the viewport,
+  but is insufficient for our needs.  We cannot simply override its properties
+  because the logic surrounding its dynamic appearance in the DOM does not account
+  for parent containers (basically, this class isn't in the DOM when we need it).
+  Intercept .usa-tooltip__content instead and nullify the effects of
+  .usa-tooltip__body--wrap to prevent conflicts)
+  */
+  .usa-tooltip__body--wrap {
+    min-width: inherit;
+    width: inherit;
+  }
+  /*
+  Add width and wrapping to tooltip content in order to confine it to a smaller viewport window.
+  */
+  .usa-tooltip__content {
+    width: 50vw;
+    text-wrap: wrap;
+    text-align: center;
+    font-size: inherit; //inherit tooltip fontsize of .93rem
+    max-width: fit-content;
+    @include at-media('desktop') {
+      width: 70vw;
+    }
+    display: block;
+  }
+}
\ No newline at end of file
diff --git a/src/registrar/fixtures/fixtures_user_portfolio_permissions.py b/src/registrar/fixtures/fixtures_user_portfolio_permissions.py
index 0ff740573..22d6a0c0e 100644
--- a/src/registrar/fixtures/fixtures_user_portfolio_permissions.py
+++ b/src/registrar/fixtures/fixtures_user_portfolio_permissions.py
@@ -1,4 +1,5 @@
 import logging
+import random
 from faker import Faker
 from django.db import transaction
 
@@ -51,23 +52,24 @@ def load(cls):
 
             user_portfolio_permissions_to_create = []
             for user in users:
-                for portfolio in portfolios:
-                    try:
-                        if not UserPortfolioPermission.objects.filter(user=user, portfolio=portfolio).exists():
-                            user_portfolio_permission = UserPortfolioPermission(
-                                user=user,
-                                portfolio=portfolio,
-                                roles=[UserPortfolioRoleChoices.ORGANIZATION_ADMIN],
-                                additional_permissions=[UserPortfolioPermissionChoices.EDIT_MEMBERS],
-                            )
-                            user_portfolio_permissions_to_create.append(user_portfolio_permission)
-                        else:
-                            logger.info(
-                                f"Permission exists for user '{user.username}' "
-                                f"on portfolio '{portfolio.organization_name}'."
-                            )
-                    except Exception as e:
-                        logger.warning(e)
+                # Assign a random portfolio to a user
+                portfolio = random.choice(portfolios)  # nosec
+                try:
+                    if not UserPortfolioPermission.objects.filter(user=user, portfolio=portfolio).exists():
+                        user_portfolio_permission = UserPortfolioPermission(
+                            user=user,
+                            portfolio=portfolio,
+                            roles=[UserPortfolioRoleChoices.ORGANIZATION_ADMIN],
+                            additional_permissions=[UserPortfolioPermissionChoices.EDIT_MEMBERS],
+                        )
+                        user_portfolio_permissions_to_create.append(user_portfolio_permission)
+                    else:
+                        logger.info(
+                            f"Permission exists for user '{user.username}' "
+                            f"on portfolio '{portfolio.organization_name}'."
+                        )
+                except Exception as e:
+                    logger.warning(e)
 
             # Bulk create permissions
             cls._bulk_create_permissions(user_portfolio_permissions_to_create)
diff --git a/src/registrar/templates/domain_add_user.html b/src/registrar/templates/domain_add_user.html
index b2f9fef24..fa3f8e821 100644
--- a/src/registrar/templates/domain_add_user.html
+++ b/src/registrar/templates/domain_add_user.html
@@ -4,11 +4,31 @@
 {% block title %}Add a domain manager | {% endblock %}
 
 {% block domain_content %}
+  {% block breadcrumb %}
+    {% url 'domain-users' pk=domain.id as url %}
+    <nav class="usa-breadcrumb padding-top-0" aria-label="Domain manager breadcrumb">
+      <ol class="usa-breadcrumb__list">
+        <li class="usa-breadcrumb__list-item">
+          <a href="{{ url }}" class="usa-breadcrumb__link"><span>Domain managers</span></a>
+        </li>
+        <li class="usa-breadcrumb__list-item usa-current" aria-current="page">
+          <span>Add a domain manager</span>
+        </li>
+      </ol>
+    </nav>
+  {% endblock breadcrumb %}
   <h1>Add a domain manager</h1>
-
-  <p>You can add another user to help manage your domain. They will need to sign
-  in to the .gov registrar with their Login.gov account.
+{% if has_organization_feature_flag %}
+  <p>
+    You can add another user to help manage your domain. Users can only be a member of one .gov organization, 
+    and they'll need to sign in with their Login.gov account.
   </p>
+{% else %}
+ <p>
+    You can add another user to help manage your domain. They will need to sign in to the .gov registrar with 
+    their Login.gov account.
+ </p>
+{% endif %}
 
   <form class="usa-form usa-form--large" method="post" novalidate>
     {% csrf_token %}
diff --git a/src/registrar/templates/domain_detail.html b/src/registrar/templates/domain_detail.html
index dca68f6ef..0fb29d2eb 100644
--- a/src/registrar/templates/domain_detail.html
+++ b/src/registrar/templates/domain_detail.html
@@ -45,7 +45,7 @@ <h2 class="text-bold text-primary-dark domain-name-wrap">{{ domain.name }}</h2>
     <div class="usa-alert usa-alert--info usa-alert--slim">
       <div class="usa-alert__body">
         <p class="usa-alert__text ">
-          To manage information for this domain, you must add yourself as a domain manager.
+          You don't have access to manage {{domain.name}}. If you need to make updates, contact one of the listed domain managers.
         </p>
       </div>
     </div>
diff --git a/src/registrar/templates/domain_users.html b/src/registrar/templates/domain_users.html
index 412f4ee73..c41902c6f 100644
--- a/src/registrar/templates/domain_users.html
+++ b/src/registrar/templates/domain_users.html
@@ -8,8 +8,7 @@ <h1>Domain managers</h1>
 
   <p>
   Domain managers can update all information related to a domain within the
-  .gov registrar, including contact details, senior official, security
-  email, and DNS name servers.
+  .gov registrar, including including security email and DNS name servers.
   </p>
 
   <ul class="usa-list">
@@ -17,7 +16,8 @@ <h1>Domain managers</h1>
     <li>After adding a domain manager, an email invitation will be sent to that user with
       instructions on how to set up an account.</li>
     <li>All domain managers must keep their contact information updated and be responsive if contacted by the .gov team.</li>
-    <li>Domains must have at least one domain manager. You can’t remove yourself as a domain manager if you’re the only one assigned to this domain. Add another domain manager before you remove yourself from this domain.</li>
+    <li>All domain managers will be notified when updates are made to this domain.</li>
+    <li>Domains must have at least one domain manager. You can’t remove yourself as a domain manager if you’re the only one assigned to this domain.</li>
   </ul>
 
   {% if domain.permissions %}
diff --git a/src/registrar/tests/test_views_domain.py b/src/registrar/tests/test_views_domain.py
index 127b78a4a..e35d38b32 100644
--- a/src/registrar/tests/test_views_domain.py
+++ b/src/registrar/tests/test_views_domain.py
@@ -340,7 +340,10 @@ def test_domain_readonly_on_detail_page(self):
         detail_page = self.client.get(f"/domain/{domain.id}")
         # Check that alert message displays properly
         self.assertContains(
-            detail_page, "To manage information for this domain, you must add yourself as a domain manager."
+            detail_page,
+            "You don't have access to manage "
+            + domain.name
+            + ". If you need to make updates, contact one of the listed domain managers.",
         )
         # Check that user does not have option to Edit domain
         self.assertNotContains(detail_page, "Edit")
diff --git a/src/registrar/utility/errors.py b/src/registrar/utility/errors.py
index cfa6a537b..e70f06d1e 100644
--- a/src/registrar/utility/errors.py
+++ b/src/registrar/utility/errors.py
@@ -23,6 +23,15 @@ class InvalidDomainError(ValueError):
     pass
 
 
+class OutsideOrgMemberError(ValueError):
+    """
+    Error raised when an org member tries adding a user from a different .gov org.
+    To be deleted when users can be members of multiple orgs.
+    """
+
+    pass
+
+
 class ActionNotAllowed(Exception):
     """User accessed an action that is not
     allowed by the current state"""
diff --git a/src/registrar/views/domain.py b/src/registrar/views/domain.py
index db0572bb3..de156598a 100644
--- a/src/registrar/views/domain.py
+++ b/src/registrar/views/domain.py
@@ -21,8 +21,10 @@
     DomainRequest,
     DomainInformation,
     DomainInvitation,
+    PortfolioInvitation,
     User,
     UserDomainRole,
+    UserPortfolioPermission,
     PublicContact,
 )
 from registrar.utility.enums import DefaultEmail
@@ -35,9 +37,11 @@
     DsDataErrorCodes,
     SecurityEmailError,
     SecurityEmailErrorCodes,
+    OutsideOrgMemberError,
 )
 from registrar.models.utility.contact_error import ContactError
 from registrar.views.utility.permission_views import UserDomainRolePermissionDeleteView
+from registrar.utility.waffle import flag_is_active_for_user
 
 from ..forms import (
     SeniorOfficialContactForm,
@@ -778,7 +782,18 @@ def _domain_abs_url(self):
         """Get an absolute URL for this domain."""
         return self.request.build_absolute_uri(reverse("domain", kwargs={"pk": self.object.id}))
 
-    def _send_domain_invitation_email(self, email: str, requestor: User, add_success=True):
+    def _is_member_of_different_org(self, email, requestor, requested_user):
+        """Verifies if an email belongs to a different organization as a member or invited member."""
+        # Check if user is a already member of a different organization than the requestor's org
+        requestor_org = UserPortfolioPermission.objects.filter(user=requestor).first().portfolio
+        existing_org_permission = UserPortfolioPermission.objects.filter(user=requested_user).first()
+        existing_org_invitation = PortfolioInvitation.objects.filter(email=email).first()
+
+        return (existing_org_permission and existing_org_permission.portfolio != requestor_org) or (
+            existing_org_invitation and existing_org_invitation.portfolio != requestor_org
+        )
+
+    def _send_domain_invitation_email(self, email: str, requestor: User, requested_user=None, add_success=True):
         """Performs the sending of the domain invitation email,
         does not make a domain information object
         email: string- email to send to
@@ -803,6 +818,13 @@ def _send_domain_invitation_email(self, email: str, requestor: User, add_success
             )
             return None
 
+        # Check is user is a member or invited member of a different org from this domain's org
+        if flag_is_active_for_user(requestor, "organization_feature") and self._is_member_of_different_org(
+            email, requestor, requested_user
+        ):
+            add_success = False
+            raise OutsideOrgMemberError
+
         # Check to see if an invite has already been sent
         try:
             invite = DomainInvitation.objects.get(email=email, domain=self.object)
@@ -859,16 +881,21 @@ def form_valid(self, form):
         Throws EmailSendingError."""
         requested_email = form.cleaned_data["email"]
         requestor = self.request.user
+        email_success = False
         # look up a user with that email
         try:
             requested_user = User.objects.get(email=requested_email)
         except User.DoesNotExist:
             # no matching user, go make an invitation
+            email_success = True
             return self._make_invitation(requested_email, requestor)
         else:
             # if user already exists then just send an email
             try:
-                self._send_domain_invitation_email(requested_email, requestor, add_success=False)
+                self._send_domain_invitation_email(
+                    requested_email, requestor, requested_user=requested_user, add_success=False
+                )
+                email_success = True
             except EmailSendingError:
                 logger.warn(
                     "Could not send email invitation (EmailSendingError)",
@@ -876,6 +903,17 @@ def form_valid(self, form):
                     exc_info=True,
                 )
                 messages.warning(self.request, "Could not send email invitation.")
+                email_success = True
+            except OutsideOrgMemberError:
+                logger.warn(
+                    "Could not send email. Can not invite member of a .gov organization to a different organization.",
+                    self.object,
+                    exc_info=True,
+                )
+                messages.error(
+                    self.request,
+                    f"{requested_email} is already a member of another .gov organization.",
+                )
             except Exception:
                 logger.warn(
                     "Could not send email invitation (Other Exception)",
@@ -883,17 +921,17 @@ def form_valid(self, form):
                     exc_info=True,
                 )
                 messages.warning(self.request, "Could not send email invitation.")
+        if email_success:
+            try:
+                UserDomainRole.objects.create(
+                    user=requested_user,
+                    domain=self.object,
+                    role=UserDomainRole.Roles.MANAGER,
+                )
+                messages.success(self.request, f"Added user {requested_email}.")
+            except IntegrityError:
+                messages.warning(self.request, f"{requested_email} is already a manager for this domain")
 
-        try:
-            UserDomainRole.objects.create(
-                user=requested_user,
-                domain=self.object,
-                role=UserDomainRole.Roles.MANAGER,
-            )
-        except IntegrityError:
-            messages.warning(self.request, f"{requested_email} is already a manager for this domain")
-        else:
-            messages.success(self.request, f"Added user {requested_email}.")
         return redirect(self.get_success_url())