I just today bought New Super Mario Bro's Wii, and it's really good. Multiplayer is awesome.
Yeah, I bought it for my birthday, and I love it. I've yet to play single player on it yet actually. Every time I start up the Wii to play it, my brother comes running in wanting to play it with me. I always let him too, because the experience is much better with someone else, which was pretty surprising to me. It takes some real genius to create a game that was previously single player only and turn into a fun multiplayer game.
Huh? If i'm not mistaken, people can join and help out with the single player mode.
There is a add/drop players option in the single player.