The acft was on a sport parachute flt to carry 24 parachutists to 12,500′ over a drop zone (dz) for a mass jump. It was in a cargo configuration with no passenger seats, but it had 1 jump seat. There were 24 seat belts on the floor of the cargo area. Also, the cabin door had been removed & an unapproved step & 4 handholds were installed outside & forward of the cabin door for parachutists. The usual jump-run procedure was for the acft to be slowed to 95 to 100 kts, extend the gear & apch flaps & reduce power on the left eng. The jumpers remained in position without using seat belts until apchg the dz, then 2 moved outside the acft while others moved close to the doorway. The 1st jumpers were unaware of anyproblem, but saw the acft enter a steep bank, roll over & spiral nose down. 16 jumpers exited the acft, but 3 hit the stabilizer. The acft crashed in a near vertical dscnt. Investigation revealed the cg was behind the aft limit for takeoff & the jump. The elevator trim actuator was found in a position for full nose-up trim.