Yes, travelling companions can also enter for free. The Senior Passes admit pass owner/s and passengers in a noncommercial vehicle at per-vehicle fee areas and pass owner + three adults, not to exceed four adults, where per-person fees are charged. (Children under 16 are always admitted free.).  Also, at many sites, the Senior Passes provide the pass owner (only) a discount on Expanded Amenity Fees (such as camping, swimming, boat launching, and guided tours).