After a few hours of reading I think to build for TestFlight I must go through the same old AdHoc distribution nightmares as usual. Set up various Certificates / Provisioning Profiles / Entitlements / etc., ask users for their UUID, register their devices, etc. etc. etc.
Then what is the benefit of TestFlight? What does it simplify?
TestFlight is primarily used (but only) for distribution of your app trough a simple and easy process by which you invite people so they can register their devices (hence, avoiding send emails with UDIDs, etc.), and receive emails with new updated builds and install with one click of a button. It also provides you an easy way to manage your builds and team members.
I don't know what I would have done with out it...