Three observers accompanied the aircrew & other passengers on a flt to transport skydivers aloft. Prior to the flt, all 3 of the observers were briefed not to exit the acft until the engines were secured. However, 1 of the 3 discussed the need to move a cessna 172 at the completion of the flt. After returning from the flt, the aircrew stopped the plane on the ramp. While the engines were still running, the occupant that wanted to move the cessna exited the acft. Unbeknown to him & the plt, the other 2 occupants also exited the acft. One of them walked into the arc of the left propeller & received a fatal head injury.