Yesterday Apple changed TestFlight to allow 2,000 beta testers and 60 day expiry on betas.
Today they've made two long overdue improvements:
- Two-factor authentication for iTunes Connect
- Ability to set per-app permissions.
The second one is especially important - although a bit late - since watchOS 2 and tvOS were not open for external testers initially.
This meant if you wanted someone to test using TestFlight (without using ad-hoc distribution), they needed to be added to iTunes Connect. This would give them access to view all apps in your account, not just the relevant one.