A "Medic" really isn't a volunteer position. This would be one of the players that would take on another role. As Fox_111 said you're best bet to get into a 18+ game would to assist the game admins in there duties.
If your really interested watch out for game in your area and contact the person who is setting up thus game. They will tell you if they need any help with anything and what you would have to do.
P.S. The guy with a camera is just a player with a camera mounted on his AEG
If ASC were real life, there would be no Chuck Norris, only CDN_Stalker