I recall on tours there were times when everyone wanted to see one small room so they were crowding to get in. I said "you have to let us out to make room" somfone said "well that makes sense. " same with elevators.
With buffet tables a smile is good. But I feel a real invitation to sit
down is the proper ettiquette. If you don't want to share that is different but if you really want to be helpful invite themm to sit down IMHO.