People sitting in a vehicle.
A group of friends are sitting in a car at a drive-in theater.
Vehicle can mean many different modes of transportation, so there is no way to infer that the group is definitely sitting in a car.  Also, the vehicle could be anywhere so nothing implies that it is in a drive-in theater as opposed to some other location.
exists e1 x1 x2 x3.(a_topic(x1) & in(e1,x3) & n_vehicle(x3) & Actor(e1,x2) & v_sit(e1) & (x1 = x2) & n_people(x1))
exists e1 x1 x2 x3 x4 x5.(at(e1,x4) & n_theater(x4) & of(x4,x5) & n_drive-in(x5) & in(e1,x3) & n_car(x3) & Actor(e1,x1) & v_sit(e1) & of(x1,x2) & n_friend(x2) & n_group(x1))