When adding email addresses for external testers in TestFlight, does the email address of the tester have to be their registered Apple ID? Or can it be any email address they can access on their iOS device?
It can be both - registered Apple ID email and not (in this case, Apple send invitation, and ask this user to create Apple id).
BUT if this email is already in iTunes Connect, you can't add this email. I faced with this problem with my client - they can't add my main email, because it is already connected to iTunes Connect account.