To the original poster: You do not need any Lawyers waiver you just need each person taking part in the even to sign a Waiver, the same as when you go to play Paintball or do any extreme sports events your made to sign a waiver before taking part that basically outlines the basic risk to safety and that the person taking part is aware of these risks and is taking part without expecting any responsibility from the Event organizers point of view. (* Depending on regional/country you may still require a permit from the local Council, and to notify emergency services so the event can be policed correctly for example.