name your passengers and use in bus driver waypoints.
you can also just pick random number or random passengers and unassign them from the crew array and make sure they are not driver instead of naming them if its no particular need for full control, creates some randomness.
place this code in bus waypoints and it will dropof 1-3 random selected units.
Code:
_null = this spawn {
_veh = vehicle _this;
_crew = (crew _veh) - [(driver _veh)];
_getout = ceil(random 3);
while {_getout > 0 AND (count _crew) >= _getout} do {
_passenger = _crew select (random(floor(count _crew)));
unassignVehicle _passenger;
_crew = _crew - [_passenger];
sleep 1;
};
};