Senior Pass Use http://store.usgs.gov/pass/senior.html

The Senior Pass admits pass owner/s and passengers in a non-commercial vehicle at per-vehicle fee areas and pass owner + 3 adults, not to exceed 4 adults, where per-person fees are charged. (Children under 16 are always admitted free.)
Note: Photo identification will be requested to verify pass ownership.

Utahtea

Thanks all for your replies. So let me see if I understand this correctly. In some National Parks your pass is valid for all passengers in the car (as debitNM) stated) and in others the pass is valid only for a total of 4 passengers? Another question: with the Senior Pass, the owner has to be the driver?
Is there a list of National Parks that indicates if the fees are per car or per person?

Some parks have a "per car fee" and in that situation, only 1 person in the car has to have a pass [any pass, not just Senior]. In those parks, everyone in the vehicle gets in [so if you have a SUV with 7 or 8 people, then they all get in for that one "fee" or in your case, one park pass].

Other parks charge a per person fee and in that situation, 4 people in one vehicle get in using the park pass.

from the nps site:

"Annual Pass Use

Can any two people sign an Annual Pass?
Yes. The two pass owners do not have to be related or married.

When does an Annual Pass expire?
Whether you buy the pass for yourself, or as a gift, the Annual Pass is valid for 12 months from the month of purchase, expiring the last day of that month.

Who is admitted with an Annual Pass?
Each Annual Pass admits pass owner/s and passengers in a non-commercial vehicle at per-vehicle fee areas; and pass owner + 3 adults, not to exceed 4 adults, where per-person fees are charged. (Children under 16 are always admitted free).

Note: Photo identification will be requested to verify pass ownership.

If I have more than four people in my car, how many passes do I need?
One pass covers the vehicle at sites that charge "per vehicle".
At "per person" sites, the applicable fee will be charged for each additional person.

My family is traveling in two vehicles; will one Annual Pass let all of us into the site?
No. Only the vehicle with the pass owner is covered. The second vehicle is subject to an entrance fee, or must have (or buy) a second pass."

from - http://store.usgs.gov/pass/senior.html

I can't find a list of which parks use which fee plan, but the individual park sites clearly indicate the fee.

The park pass can also be used at other facilities [see the above link].

The rules above are for ALL park passes, not just senior.

Thanks again DebitNM. Should have checked beforehand the NPS site. All my questions has been answered to my satisfaction.