I added a tester to the group, then realised I had entered the wrong name.
You can't rename a tester so I decided to delete the user from the group and then add them again with the correct name.
However, the old name displays. It isn't at all possible for me to get them under the correct name...
Try changing the email attached to the old name...make something up, etc., then wait a bit and see if you can add a new user with the correct name and email.