Use the 'invite' command to invite another player to your party.
If that player accepts, via the 'acceptinvite' command, they and any
splitscreen players on their machine will be added after the players
in your party. They may also use the 'rejectinvite' command.
Use the 'leaveparty' command to leave a party. You and any splitscreen players
on your machine will be removed from the party. Players after you will be
shifted to take your place on the splitscreen.
