A person whose job it is to facilitate a good time while at the same time preventing people from ending up in either jail or the hospital.
A secondary function of the lifeguard is to provide a supplement to facebook memory by providing a coherent record of the events of the party to everyone else the following morning. Thus providing guests with a person to whom they can pose the question "What the fuck did I do last night?"
Jay: "Greg's got it"
Nick: "I'm jonzing for some fuckin' wings"
Jay: "dude, get the lifeguard to order some"
Nick: "What the fuck did I do last night?"
Lifeguard: "You tried to jump off the fucking balcony 'cause you thought you could fly"