There's already a thread for this issue https://developer.apple.com/forums/thread/702988. Bug was reported over 6 months ago and there's no resolution yet.

I submitted a bug report through the Feedback Assistant app. This is the first response they gave: Thanks for your report. We need additional information on this TestFlight issue.   First, please try the following troubleshooting: Remove the affected testers from all Internal and/or External tester groups Remove the affected testers from the All Testers tab under General Information  Reinvite the testers to the correct group(s) If the issue persists, please provide the App ID of the app you are trying to add testers to. Thank you. I tried the above steps but it didn't help. This happens with all of our apps.

I have the same issue. I can't add anymore testers to our internal test group. If I remove an existing user from the group, I can add that user back, but no new users are allowed.