You cannot use the senior pass for concessions within the National Parks. Trailer Village in the Grand Canyon and Fishing Bridge RV Park in Yellowstone are not eligible for campground discounts as they are run by outside concessions. Also the parking at Mt. Rushmore is run by a concession so the pass does not work there.
We have used our passes to get our children and grandchildren into National Parks with us. I believe the pass covers 4 people, the holder and three others. Both DH and I have a pass so we can take 6 additional people with us.