From my experience, you have to call in to get this done or do it at the airport. I've found that some agents simply don't know how to do this. On a recent trip, my traveling companion was on a different PNR. I called ahead and the person I talked to said that everything was taken care of. But after checking in, my companion wasn't listed with me. At the airport, the agent said that she couldn't do this if we weren't on the same PNR (incorrect). In my connecting city, I went to the Admirals Club agent and the agent was able to do what two others could not do. She told me that in order to inherit the status, they remove the FF# of the companion, which then moves the companion to your status, and then they add back in the companion's FF#. That seemed to have worked, as my companion received the appropriate upgrades and miles for the trip.