Pilot of a skydiving plane reported loss of control; a stall and recovery after the aircraft CG was upset by too many jumpers crowding the jump door at the same time.

Narrative

On Date I was operating Aircraft X for jump operations at ZZZZ.Prior to the incident I (PIC (Pilot in Command)); was told that 13 Skydivers would be practicing for a big way formation jump. There was a minor brief with the Coordinator for the formation jump wherein he advised me that they were planning on having up to 10 Skydivers hanging off of the jump step and in the door situated aft and on the left side of the aircraft. I advised the Jump Coordinator of the following:'This won't work and it's not a good idea. Here's what I mean - If we use a weight estimate of 170 lbs. per skydiver; multiplied by 10 bodies; we get a conservative estimate of 1700 lbs. aft of the center of lift or balance point. This will put the aircraft well out of the CG limitations. If this happens I'll lose pitch control of the aircraft and there's a very good chance the aircraft will stall; and possibly go into a spin.' The Jump Coordinator responded: 'OK; we'll figure out a different configuration.'My response: 'Listen; if your guys don't get this right this bird's going to drop a wing; stall and there's a good chance the aircraft will enter a spin right on top of you guys.'The Jump Coordinator though that I was joking as he responded with a laugh. I replied with a reaffirmation of my previous statement: 'hey man; I'm dead serious! If this happens and that wing drops; you guys need to get the hell out of dodge quick because there's a good chance the plane's coming right on top of you!'Prior to the skydivers boarding I warned my Trainee (who was observing on this flight from the right seat):'If these guys screw this up we're going to get some spin recovery practice... I'm hoping this won't be the case but I'm going to plan for it and carry extra speed as well as trim the elevator forward on the jump run. This way; if we get out of CG and I start to lose pitch authority; it'll buy us a few extra seconds. Just be ready because these guys lose 98% of their brain cells once they get around airplanes... Point being - let's hope for the best but expect the worst.'Takeoff and climb out were normal with no anomalies. At 12;500 ft. I flashed the standby red-light in the back of the aircraft. Two miles prior to the exit point; I turned on the red standby light which gives the 'OK' to open the jump door. 1 mile prior to the exit point; I configured the airplane with 20 degrees of flaps; pulled the power back to 1000 lbs. of torque; pulled the propeller back to the 1st detente; trimmed the nose forward; and put the aircraft in slow flight pitching for an 80-85 kt. airspeed (roughly 10 kts. faster than usual) .From there I turned on the green light signifying to the jumpers that they were clear to exit the aircraft. Immediately; the skydivers began moving from the front of the aircraft and positioning themselves on the step outside of the airplane. A few seconds past when I told my Trainee: 'look; I'm full forward on the yoke; I have no more pitch authority and I'm out of of elevator trim. At that very moment two more skydivers rushed to join the group already at the rear of the airplane. The airplane began to pitch up and yaw to the left. I immediately came in with a slight amount of right/opposite rudder to bring that retreating wing forward; and back into the relative wind. About two seconds later the left wing stalled and the aircraft rolled aggressively into a roughly 80 degree bank to the left which threw the skydivers off of the outside step. Once the skydivers were off of the plane; control was restored back to the pilot (me); I rolled the wings level; took out the rudder input and recovered from the stall and leveled the aircraft off in order to stop it from accelerating past 130 kts.From there; I made my descent to land and all else was fine.