The invitation system is so annoying, i stopped using it and play rather alone than to invite other players. You need to send messages through the PS messaging system, after that, the one you have invited needs to switch out of the game, accepts the invite, goes through a loading screen again just to realize that he has been ported back to the last checkpoint (or beginning of the map). In a game that has such a strong focus on multiplayer, why can't you just invite people ingame ? Options Menu -> Invite Player nearby -> finish.