Make a repeating trigger, or a script-snippet with a loop, with the following condition:
vehicle playerUnit != playerUnit
And put the following in OnActivation(/or later in the script):
"_x assignAsCargo vehicle playerUnit" foreach units group playerUnit ; units group playerUnit orderGetin true
(Syntax not guaranteed)
What it does is first check if the player's vehicle is not itself, and if so, assigns everyone in the players group as cargo (including the player, but that probably wont be a problem, probably. . . ), and finally orders them to get in.