We had this issue. Description: Tried to move the tester to a new group where a more recent build was available, but "View in testflight" from the invitation would only give him the build from the previous group and was showing as "Invited" in Testers for that group. I tried giving the link to the build with the same result. I removed the user from all groups and now the app inside TestFlight was showing as "User removed from testing" (or something similar). Solution: The user has to go on TestFlight, click on the app which should still be visible and click on "Stop testing". It will then get removed from TestFlight and can accept new invitations.

same!