Unlike PSO, PSU's multiplayer focuses exclusively on completing missions. You can get 'em in a variety of places, but they all boil down to beaming into hostile dungeons and smacking the crap out of hordes of mutated monsters. Your party leader's responsible for choosing the mission and setting loot rules, which will ensure that everyone gets a piece of the profits. Once everything's set, it's into the fray. If you've got an open member slot, the party leader will have the option of bringing their partner machine along to fill in. Depending on how they raised it, it'll have various offensive or defensive behaviors. The most important missions will push the game's online story along, chronicling the events that occur after the conclusion of Ethan's single-player adventure.
So, that's the lay of the land so far. The game'll launch on all platforms on October 24, and an Xbox Live beta test should begin as early as next week. PSO fans, your long wait's almost over.