RE: National Park Entrance Fees
I think the confusion comes in the differences from park to park.  Some
National Parks charge a 'per vehicle' fee, other charge a 'per person' fee.
The passes are limited to four people total in the 'per person' parks,
unlimited number of people (in one vehicle) in the 'per vehicle' parks.  At
least that's the way I read the rules on that page.