Unfortunately I have never found a good way to do this. I just make sure that when dining with another reservation they go in and book their reservation for the same time and date, then as soon as I get on board the ship I let the restaurant know that the two or three reservations/ stateroom numbers are dining together. Usually all they want is stateroom numbers. We have never had a problem doing that, even if someone in the group could not get the same dining time.