I was going to go even simpler and say turn the games on, let people play as they please and explain a few basic goals to those that want to know more. I don't know many non-pinball people that would have the patience for a 20 person tournament and have to be committed all night. We do a Christmas party every year and only one of my other friends is really into pins. Other people will play them but I just turn everything on and let people come and go as they please. Most people seem to go "oh cool pinball!", play one or two games and then come back to the party. The less pressure the better IMO.