Well you're gonna need someone to hold the frontline, so a fighter, paladin, or warden would be a good place to start. Next you're gonna need someone to keep them alive, so a cleric or priest is a good idea. And since surviving isn't going to progress through the dungeon, some damage dealers are also needed if you want the party to get anywhere. A rogue and a wizard are the classic choices, but warlocks, barbarians, rangers, psions, and druids do the job just as well. If you want it to be bigger, you're gonna need more of everything but groups of 4 are the best to work with.
OT: A group of people, some entertainment, and something to eat/drink is all I figure. I've had tons of fun just playing games (anykind will do) or watching movies with friends.