If you invited yourself as an 'external' tester via a different email address, you can forward that email to the actual beta tester. It states:
You can accept this invitation with the Apple ID that you are currently using. Your Apple ID and the email address to which the invite was sent don't have to match.
Caveat: back in iTunes Connect, YOUR email will show up as having 'Accepted' the invite, even if the Apple ID of the accepted device is associated with the original beta tester's email.
