I am not sure if I have understood your problem, but if a group leaves a helicopter with get out waypoint it does not abandone the helicopter, it is still in possession of the helicopter. If you try to get into the heli with another group it will not work because AI considers to have no permission of getting into a helicopter of another group.
To fully abandone a helicopter you need to give an extra command in the On Activation field of the get out waypoint as: this leaveVehicle vehicle;
This allows other units to get into the helicopter, which was used by another group before.
