How to use same TestFlight invitation from different devices? I want to test beta app with different devices, but when I open app from mail in TestFlight and try to start it refuse and shows "The invite has already been used". Any workaround?
As Janos said, 
In order to get an invitation code again, the hack is to remove that email account from being a tester and re-add him again.
Apple will send him an email with a new redeem code.

Otherwise, Apple will send an email without a redeem code.