This answer varies and it can be determined once you know how the sites you are visiting charge their entrance fees. Larger groups only need one pass if you visit sites that charge per vehicle. Children under 16 are always admitted free.
For sites/parks that charge PER VEHICLE - up to 15 people can enter the park if they are all riding in a non-commercial vehicle and the pass holder is in the vehicle.
For sites/parks that charge PER PERSON - up to 4 adults can enter the park with one annual park pass which includes the pass holder.
What is the difference between park charges per vehicle and per person?
Non-Commercial Vehicle: Passenger car, van, pick-up truck, converted school bus, recreational vehicle, vehicles with trailers, etc., used to enter a Federal recreation site for non-commercial purposes.