1 Volunteer and 3 helpers
Have people find out the weight of the volunteer prior to the game.
(They need to find out as naturally as possible). During your meeting have one of the helpers come up and explain how this person has the uncanny gift of guessing the weight of anyone they meet. Then ask for three volunteers. Choose the other two helpers and the volunteer you are going to get. Have the three 'volunteers' stand off to one side of the room. Have the weight machine person sit on the floor and then bring over the first helper/volunteer. They sit down facing the weight machine and put the bottoms of their feet against the bottom of the weight machine's feet. They both reach across and grab wrists. The weight machine leans back and lifts the other person off their butt and then lowers them back down. They think about how heavy the helper was and then announces their weight. (Of course, you already gave him this information beforehand - although a pronouncement of 400 lbs for a small ninth grade girl is funny, too) This is repeated for the other helper and then the true volunteer. But after the weight machine lifts the true volunteer up you slide a monster sized, plastic bowl or pan of water under where their butt will be lowered.