I had an issue with this when staying in Hilton Amsterdam Airport Schiphol with my colleague in two rooms both booked by me.

Basically the discussion went: "The breakfast is only for the guest staying in your room" - "Ok. nameofmycolleague is the 2nd guest in my room." - "But who is staying in the other room?" - "It's for our luggage" - "Unfortunately it's not possible as..."
And naturally they required the 2nd guest also to register there at the hotel in person so putting "imaginary person" wasn't an option.

In any case they didn't track guests who are eating breakfast in the Executive Lounge, only in the restaurant. The Executive Lounge breakfast there has a very good full breakfast selection also.