I am having the exact same problem. I then did a new invite to myself and when I went to accept it, it said "TestFlight is currently unavailable. Try again." However, the System Status on Apple's dashboard isn't showing any issues. Hmmm....

I am experiencing the same issue with my app -- no users can sign up or sign back in. Since the identity token is blank -- I can't decode and verify. Hopefully gets fixed soon... I've noticed it for the last hour.