Anyone can sign up ("request invite"). These showcases are open to anyone and everyone... Invites are just a way to generate awareness in the hopes that more sign up.
The only real "invite-only" events are:
PG National
PG Underclass All-American
PG All-American (of course...)
There are quite a few where you can receive an invite that is selective (High ranking, high grade from previous event, etc.). But you can actually request an invite to these as well (as long as there are spots open):
PG Jr National
PG Underclass All-American
Various PG Top Prospect Events