Question - how many GCN controls do you have - 'cause if so there's quite a few GCN games to look out for for a fun multiplayer. I'm a big fan of kirby air ride (say waht you want about the puffball, city mode in air ride is an excellent place for epic scales of douchebaggery, something that's allways fun in a group), Metal Arms and timesplitters 2 on the GCN.
But if you're sticking with solely Wii games then the only thing i can offer from my library (other than brawl and mariokart), as a "gathering game", is Zack and Wiki. It's 2 player mode basically lets another person write stuff on the screen to help guide you, so you have 2+ people trying to solve a puzzle. Other then that...wario ware? Your mates might not go for that (or, if they do, its to mock the person that gets the hula hoop and dancing minigame).
um..stuff
