or, if that doesnt work, try making a trigger to activate when you want them to get out. name each guy, man, mana, manb etc. and put in on activation field of trigger:
unassignVehicle man; unassignVehicle mana; unassignVehicle manb; etc.
if THAT dont work, try naming the truck and putting the name in place of 'Vehicle'.
there is porbably a way of doing doing it without having to type it all out, as in "_x moveincargo foreach units GROUPNAME" but i dont know what that is.
unless it is simply "_x unassignVehicle foreach units "GROUPNAME", but i dont know.