This answer varies and it can be determined once you know how the sites you are visiting charges their entrance fees. 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.
List of Parks: https://www.usparkpass.com/list-of-us-parks
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.
Please visit the following page for an explanation of entrance fees & if you travel with more than four passengers: