Update CONTRIBUTING.md

We were blocked on adding new contributors to #expensify-open-source cuz of limited Single Channel Guest (SCG) seats.  Niki removed a bunch of inactive users so we have seats now!  
Expensify/Expensify#425298

contributingGuides/CONTRIBUTING.md

@@ -31,6 +31,15 @@ This project and everyone participating in it is governed by the Expensify [Code
 ## Restrictions
 At this time, we are not hiring contractors in Crimea, North Korea, Russia, Iran, Cuba, or Syria.
 
+## Slack channels
+All contributors should be a member of a shared Slack channel called [#expensify-open-source](https://expensify.slack.com/archives/C01GTK53T8Q) -- this channel is used to ask **general questions**, facilitate **discussions**, and make **feature requests**.
+
+Before requesting an invite to Slack, please ensure your Upwork account is active, since we only pay via Upwork (see [below](https://github.com/Expensify/App/blob/main/contributingGuides/CONTRIBUTING.md#payment-for-contributions)). To request an invite to Slack, email [email protected] with the subject `Slack Channel Invites`. We'll send you an invite! 
+
+Note: Do not send direct messages to the Expensify team in Slack or Expensify Chat, they will not be able to respond. 
+
+Note: if you are hired for an Upwork job and have any job-specific questions, please ask in the GitHub issue or pull request. This will ensure that the person addressing your question has as much context as possible.
+
 ## Reporting Vulnerabilities
 If you've found a vulnerability, please email [email protected] with the subject `Vulnerability Report` instead of creating an issue.