Square-Enix is definitely trying to be a bit misleading, here, in this particular situation.
The Quest: Go online and upload a save file. The multiplayer aspect: If 500,000 players upload a save file, you get a reward. The reward: A character from another game makes a cameo appearance.
There isn't "team" gameplay in this particular quest. There's virtually no gameplay in it, actually.
But, don't let me mislead you, either... This is just the initial quest, and there certainly could be a lot more coming that's a lot more interesting. For example, imagine a "world-wide" boss, someone that everyone can fight, one-on-one (or four-on-one, if participating in local wireless play,) and this boss has a massive HP bank that decreases every time you upload a file.
Yeah, it's not the most exciting thing, but there's potential here, regardless. The problem is that SE is definitely painting a different picture than what's actually been shown so far.