Can i be sued if men in my yard get hurt doing contact sports?

My 19 yr. old son and his friends want to 'work out' in my front yard. They do ultimate fighting, which is a contact sport, and jujitsu. They will also be working out using small weights, an exercise ball, and a punching bag. He says that as long as they sign a paper saying that I'm not liable if they get hurt, I can not be sued. Is this true? Would the release form need to be stated in legal language? How might I go about finding out legal implications in this situation?