I don't understand yet how TestFlightApp manages to send my app to any tester I invite by email. Actually AdHoc Distribution is limited to 100 devices and here and there I read about Enterprise signed IPAs for no-limit beta testing.
So I need the enterprise account?
no, you do not. It works fine with a regular developer account. I use it all the time.
You are limited to 100 testers though, TestFlight does not get around that.