Just tried the party stuff and it is pretty odd.
Started an Xbox Live Party and booted the game.
Started a Private game but had 2 human slots available.
Once the game starts, you end up in a warmup lobby.
From the menu, you can send out invites to those in your party.
Once the match starts, you can convert to the same FireTeam and swap sides (If you end up on odd sides)
Seemed to work ok but not as simple as it could be. They should give players the option to start an Xbox Live Party and then have that show in the Brink menu's, like the Dashboard menu does.