I just received an email this morning that testflightapp.com will no longer be active as of 2/26/2015, and that I should be using the iTunes Connect TestFlight service inste
Tesflight is not available for iOS Enterprise development. See "If you’re distributing your app outside the store, you follow a slightly different process. You don’t have access to iTunes Connect and some app services so can skip those steps." at https://developer.apple.com/library/ios/documentation/IDEs/Conceptual/AppDistributionGuide/Introduction/Introduction.html
Also see Apple discussion at https://forums.developer.apple.com/thread/9229 where it says "Enterprise apps can be tested internally using Ad-hoc distribution" if one wants to only have the app available to registered test devices.