I have a beta tester signed up for TestFlight. They originally accepted the invite using both the wrong Apple ID and wrong device. We want to change the device and Apple ID they are using.
I had them open TestFlight on the wrong device, select our App and select "Stop Testing". I've also removed them from the test group.
I've then waited for 24 hrs and re-added them to the test group using the same email as before. Their status is "Invited".
However, they keep getting the error
Couldn't redeem TestFlight invitation code "<code>" This invitation has been revoked or is invalid. Request a new invitation from the developer.