Since no one else has chimed in, I will.
Invitations are usually extended to the lucky few who are considered, by the host casino, to be worthy of such grandiose treatment because of past play history in that casino. Different places have different levels of required play.
And almost all of them have a "don't ask - won't tell" policy about those required play levels.
It is one of the two cancers preventing any growth in the game's popularity (UIGEA being the other).