My son got invited, and have no idea how. We didn't attend, which is now a big regret of mine, as we had no idea what it was and the email/web site was not very impressive.
I only mention that, as we did not fall into the above categories. My son only played travel ball summer of his rising senior year, and it was a small, local team. He's a RHP, but not throwing 92.
I'm sure the above are accurate, and maybe we were the exception. But don't think I'll ever know how he got the invite.
Just searched for the email, which was titled Area Code/East Coast Pro Tryout. My guess is that's the same thing.