Similar to the taco idea but you can make pizzas or burgers and just cut up all your different toppings and have people make their own - just buy pizza bases or use pita bread if you don't want to make your own.

Otherwise, if I'm cooking for a crowd, I generally go with a roast or I make a couple of lasagnes (usually one vegetarian and one for meat eaters) and just serve with salads and garlic bread.