The pilot started a downwind takeoff on runway 15 with 9 parachutists on board. The runway was unidirectional requiring takeoffs on runway 15 which sloped downhill. After beginning the takeoff roll, the pilot had difficulity in maintaining directional control and initiated abort procedures. Reportedly, the aircraft continued to veer to the right in spite of the use of hard left rudder and braking action. As the aircraft decelerated, it departed the right side of the runway, struck 3 fence posts, and collided with a parked stinson, n368c. The pilot reported that the parachutist occupying the copilot’s seat stated that he had placed his feet behind the rudder pedals shortly before takeoff.