This is good for all ages.
1. Have the large group stand in a circle.
2. Have a leader call out a body part and a number. (ie. 5 elbows)
3. Everyone has to immediately get into groups of five with their five elbows sticking together in the center. Keep doing this with different body parts and numbers. The last one should be a ridiculous number with a ridiculous body part (ie. eyelashes). No one will do it and they will probably sit back down. They now know the game is over.