42:20 If a group of people eat together and some of them leave with the intention of returning, as long as at least one person remains in the original place, the original gathering is not considered as having been disbanded. When the others return, they are considered to be returning to the original gathering without interruption.
42:21 If one has the intention while saying hamotzi that he will relocate during the course of the meal, the practice is to permit this. One must be careful to eat at least an olive-sized piece of bread in the new location. This should really only be done when needed for a meal of a mitzvah.