Here is how you can do it, make a script called
para_jump.sqf in your mission folder, and put this code inside it:
{
_unit = _x select 0;
moveOut _unit;
sleep 0.2;
_chute = createVehicle ["NonSteerable_Parachute_F", getPos _unit, [], 0, "CAN_COLLIDE"];
_unit moveinDriver _chute;
sleep 0.2;
} forEach fullCrew [_this select 0, "cargo"];
Then put this code in the init of the plane you're using:
this addAction ["Initiate Jump", "para_jump.sqf"];
This will add an action to the plane that will make everyone in the cargo seats jump out, and it will give them parachutes after they jump. It has small time intervals so nothing collides with each other.