It's not pay to win, it's rather a pay to advance model; a player receives extra time to progress within the game, instead of providing a player with enhanced weapons that are not given as rewards ingame. You can get just as good as that player within the same amount of time.
On the invitational part; I'd suggest you create the accounts by hand for the selected players. You can also get your AAC modified, but that'd require a programmer. Also, I'd go with an key-like invitations, if you're planning to integrate it into the AAC.