I've been able to "guest" in a companion who was on a different PNR and would not have had access otherwise. The person at the stand outside did not really ask though, so I don't know how official that was. All they did was find my name on their list and let us in. Once inside, the ticket agent checked me in and then I said my companion is on a separate reservation but the same flight, and they gladly checked them in as well. The escort took the elevators up with us and guided us to cut the pre-check line where Clear members normally enter.