Oh, and Donkey Konga is great with two players or more. You can pick up a lot of the equipment inexpensively now too. By far the most popular multiplayer game at my college, particularly because everyone will play the game. That includes girls, guys, parents, kids, non-gamers, gamers.
Oh and don't worry about the comment that you have to "unlock" stuff. All the modes and songs are available from the beginning. The stuff to unlock is minimal.