this is what you want:
unitname assignAsDriver vehiclename;
[unitname] orderGetIn true;
You can also use the
MoveIn command
unitname moveInCargo vehiclename
unitname moveInDriver vehiclename
unitname moveInGunner vehiclename
but it is more of a "jump" into vehicle if you will see the action, rather than the more realistic version Gundy noted
You may also need to utilize the
UnassignVehicle command when using these to get your units out
UnassignVehicle UnitName
or "unassignVehicle _x" foreach units thislist ....don't remember syntax :-X