Use the medicine ball as the volleyball and score the game the same way you would if you were playing sand volleyball. Instead of bumping setting, spiking, you will catch the medicine ball in proper form allowing your knees to bend (without hunching your back) and you will return the ball by throwing it, any way you can, over the net to the other team. Points are made when the ball hits the ground.
This game becomes a very fun and competitive way to workout and bond with friends. Just be careful to catch an throw with proper form. Make sure, before you start, to have someone demonstrate how to catch the weight of the ball without getting injured. Have fun!