diff --git a/app/controllers/admin/members_controller.rb b/app/controllers/admin/members_controller.rb
index 82f3f7b06..2147aed58 100644
--- a/app/controllers/admin/members_controller.rb
+++ b/app/controllers/admin/members_controller.rb
@@ -75,7 +75,6 @@ def create
       impressionist(@member, 'nieuwe lid')
       redirect_to(@member)
     else
-
       # If the member hasn't filled in a study, again show an empty field
       @member.educations.build(id: '-1') if @member.educations.empty?
 
diff --git a/app/controllers/api/calendars_controller.rb b/app/controllers/api/calendars_controller.rb
index 49d713326..3141a5f2e 100644
--- a/app/controllers/api/calendars_controller.rb
+++ b/app/controllers/api/calendars_controller.rb
@@ -16,7 +16,7 @@ def show
     # end
     respond_to do |format|
       format.ics {
-        send_data create_calendar,
+        send_data create_personal_calendar,
         type: 'text/calendar',
         disposition: 'attachment',
         filename: "#{@member.first_name}_activities.ics"
@@ -36,10 +36,21 @@ def index
   end
 
   # Not exposed to API directly, but through #show
-  def create_calendar
+  def create_personal_calendar
     @member = Member.find_by(calendar_id: params[:calendar_id])
-    locale = I18n.locale # TODO werkt dit?
-    events = @member.activities.map { |a| IcalendarHelper.activityToEvent(a, locale) }
-    IcalendarHelper.createCalendar(events, locale).to_ical
+    @locale = I18n.locale # TODO werkt dit?
+
+    # Convert activities to events, and mark activities where the member is
+    # is enrolled as reservist
+    @reservist_activity_ids = @member.reservist_activities.ids
+    events = @member.activities.map do |a|
+      if @reservist_activity_ids.include? a.id
+        a.name = "[RESERVIST] #{a.name}"
+      end
+      IcalendarHelper.activityToEvent(a, @locale)
+    end
+
+    # Return the calendar
+    IcalendarHelper.createCalendar(events, @locale).to_ical
   end
 end
diff --git a/app/models/member.rb b/app/models/member.rb
index 9b5199843..716a34b91 100644
--- a/app/models/member.rb
+++ b/app/models/member.rb
@@ -16,6 +16,7 @@ class Member < ApplicationRecord
   validates :emergency_phone_number, presence: true, if: :underage?
 
   validates :email, presence: true, uniqueness: { case_sensitive: false }, format: { with: /\A.+@(?!(.+\.)*uu\.nl\z).+\..+\z/i }
+  validates :calendar_id, presence: true, uniqueness: true
 
   # An attr_accessor is basically a variable attached to the model but not stored in the database
   attr_accessor :require_student_id
@@ -203,6 +204,12 @@ def groups
     return groups.values
   end
 
+  # We cannot enforce that a member is created with a calendar_id, but we can enforce that
+  # when a member is created or saved, a calendar_id is set before validation.
+  before_validation on: [:save, :create] do
+    self.calendar_id = SecureRandom.uuid if self.calendar_id.blank?
+  end
+
   # Rails also has hooks you can hook on to the process of saving, updating or deleting. Here the join_date is automatically filled in on creating a new member
   # We also check for a duplicate study, and discard the duplicate if found.
   # (Not doing this would lead to a database constraint violation.)
diff --git a/db/migrate/20240521180025_add_calendar_id_to_member.rb b/db/migrate/20240521180025_add_calendar_id_to_member.rb
index da7f4da90..d9fa39ca1 100644
--- a/db/migrate/20240521180025_add_calendar_id_to_member.rb
+++ b/db/migrate/20240521180025_add_calendar_id_to_member.rb
@@ -1,5 +1,19 @@
 class AddCalendarIdToMember < ActiveRecord::Migration[6.1]
-  def change
+  def up
     add_column :members, :calendar_id, :uuid
+
+    # Populate existing records with a random UUID
+    Member.reset_column_information
+    Member.find_each do |record|
+      record.update_columns(calendar_id: SecureRandom.uuid)
+    end
+
+    # Add a unique index to the UUID column, because why not
+    add_index :members, :calendar_id, unique: true
+  end
+
+  def down
+    # Remove the UUID column
+    remove_column :members, :calendar_id
   end
 end
diff --git a/db/seeds/members.rb b/db/seeds/members.rb
index 9b51941fc..145c314c8 100644
--- a/db/seeds/members.rb
+++ b/db/seeds/members.rb
@@ -19,7 +19,7 @@
     birth_date:   Faker::Date.between(from: 28.years.ago, to: 16.years.ago),
     join_date:    Faker::Date.between(from: 6.years.ago, to: Date.today),
     comments:     (Faker::Boolean.boolean(true_ratio: 0.3) ? Faker::Hacker.say_something_smart : nil),
-    calendar_id:  Faker::Boolean.boolean(true_ratio: 0.4) ? SecureRandom.uuid : nil
+    calendar_id:  Faker::Internet.uuid
   )
 
   puts("   -> #{ member.name } (#{ member.student_id })")
diff --git a/db/structure.sql b/db/structure.sql
index 1ab61df75..afccb6e24 100644
--- a/db/structure.sql
+++ b/db/structure.sql
@@ -1450,6 +1450,13 @@ CREATE UNIQUE INDEX index_group_members_on_member_id_and_group_id_and_year ON pu
 CREATE INDEX index_impressions_on_user_id ON public.impressions USING btree (user_id);
 
 
+--
+-- Name: index_members_on_calendar_id; Type: INDEX; Schema: public; Owner: -
+--
+
+CREATE UNIQUE INDEX index_members_on_calendar_id ON public.members USING btree (calendar_id);
+
+
 --
 -- Name: index_members_on_email; Type: INDEX; Schema: public; Owner: -
 --