For the most part, those that attend are Army SF, Rangers, LRSD soldiers, Navy SEALs, Navy EOD, AF PJ's, CCT's, SWO's, AF SERE instructors, USMC Recon (bn and force), parachute riggers, ALLIED nation troops, and selected DOD civilians, et al. However, MFF (Military free fall) is not restricted to SOF personnel.
If you're in the Marine Corps, you can forget it. They aren't in the habit of sending people to schools unless there is a need.
The Navy is pretty much the same way, but I believe you were once asking about EOD (correct me if I am wrong) and MFF is part of the pipeline training.
The Army will occasionally offer it as a re-up bonus, so they are by far the easiest to squeeze your way in with. But just because you go to the training, doesn't mean it will be anything more than a bolo badge. Very few units need this capability.
AF mainly sends their SOF fellas. Mark may know more about their end.