A commercial plt was flying a group of parachutists for a skydiving club. The aircraft encountered turbulence during a climb to 4,500 feet agl. As the second jumper was exiting, the aircraft dropped and the parachutist struck the horizontal stabilizer. The outer four inches were bent downward and the assembly was pulled one inch from the fuselage. The plt made a normal landing and an inspection revealed some bulkhead damage in the tail section. The parachutist was not injured.