Post not yet marked as solved
Click to stop watching this thread.
You have stopped watching this post. Click to start watching again.
contentPostList.repliesup-voted.tooltip
I had the same problem.
Open the invitation email on a device that DOESN'T have TestFlight installed.
Tap on the blue "View in TestFlight" button and you will be taken to a testflight.apple.com page which provides a redeem code.
Go to TestFlight on your test device, tap "Redeem" and enter the code.