Try this.
Just make it a move waypoint, and in activation type:
unitA assignAsDriver car;{_x assignAsCargo car} forEach [unitB,unitC,unitD,unitN];[unitA,unitB,unitC,unitD,unitN] orderGetIn true
Replace unitA with the name of your driver and unitB,unitC,unitD, etc. with your other units. If you only have one other unit apart from the driver, you don't need the {}forEach in there; just erase the array and change _x to the name of the unit that you want to get into the back.