We played this game at youth group conventions and it was so much fun!
All participants make a large circle each sitting in his/her own chair.
The leader will give a command like "If you have brown hair, move two places to the left." So those with brown hair will move and sit (or pile up) on those who are sitting.
You can use other commands that refer to grade level, birth month, age, etc. But the fun part is watching all the people pile on top of each other.
Extra chairs can be taken away throughout the game to make sure everyone is participating in the pile up.