The party system seems to do a better job of keeping you together than I've ever experienced in Red Dead Redemption. not knocking the game (Still planning on going back to it and finish the online shit), but the only time you're routinely broken up is when you join a game in progress.
Yesterday afternoon, I was playing in a party for like 5 games in a row, and we were always on the same team except for that first one in progress. Usually only play this in short bursts, but you take the point.
It was funny though. the guy who got seeded to the other team got ganked by all of us. We weren't really teaming up on him or anything...Hell, when I backstabbed him, I didn't know it was him. Oops.
He was brand new to the game though, and will probably be better than me by...Sunday.