While working with iOS 7, we never had these issues, but since iOS 8 has come in, we have noticed a lot of problems with our testers.
With iOS 7, whenever new device/tester joined our team, we would create a new provisioning certificate with apple, and update it on testflight. It would seamlessly let iOS 7 users know that the build was updated, and they were easily able to use it.
But now when a iOS 8 user tries to download the build, the error that he gets is, as if his UDID was never added to the build, although in provisioning certificate, it is clearly listed.
I have seen a couple of questions related to this but couldn't find a clear solution.
Currently the workaround I am doing is, everytime a iOS 8 user joins in, I have to create a new build using the new provisional certificate, and upload it again on testflight.
Its getting worse now, because number of our testers is increasing day by day.
I am using XCode 5.1.1 (5B1008) to create the builds.
Since Apple acquired TestFlight, they changed the whole process for iOS 8:
I have successfully replaced my old process by following this one: http://code.tutsplus.com/tutorials/ios-8-beta-testing-with-testflight--cms-22224