If you do things physically, you can do it easily.
For example you could have a hat with pieces of paper in it each with a number. The numbers sum to 0 (some are positive, some are negative). Each person draws a piece of paper, adds his or her weight to it, then says that number. The numbers that each person says are summed and that's the total sum.
For example you could have a hat with pieces of paper in it each with a number. The numbers sum to 0 (some are positive, some are negative). Each person draws a piece of paper, adds his or her weight to it, then says that number. The numbers that each person says are summed and that's the total sum.