The way I understand it, extra vehicles park in the day lot, and must display the site pass. Each day the ranger relays the number of available spaces to the gate, and it goes from there.
The fee for everyone but those lucky enough to live in the Kingdom of Maine is now $12 per car. BTW, if you have a Maine resident in your vehicle, and they have proper ID, your car gets in free, regardless of the color of your plate (unless you happen to have a Mass plate- then it's double
) Also, make sure to let the Maine resident out of your car before you go hiking
Oh yeah, one last thing. Make ABSOLUTELY certain that each late arriver has a copy of your reservations. Otherwise they will not be allowed to enter the park. It's also a good idea to tell the rangers that they are coming. It is strongly suggested that all campers arrive at the gatehouse before 8:30 PM (see regulations on camping)
Here (#7). if you are going to be later, call ahead to get the OK. Too darn far to leave anything to chance!
You won't find nicer Park Rangers anywhere - have a blast!