Siman 479

And after he has eaten the afikoman – they should pour him a third cup, and he shall recite Birkat HaMazon over it. And they wrote that the cup requires rinsing and washing, even for one who is not careful regarding this during the rest of the year. However, nowadays we clean the cups for the first cup, and with it we drink all four cups. And it is customary to seek out a zimun for the Seder, and not because of the reason of Birkat HaMazon, but rather because of Hallel for "Hodu" and "Ana," for which one must read and two must respond, as it is found in Midrash Tehillim. However, if he does not have a zimun – behold, his wife and his sons and his daughters respond after him (Tur).

This is the wording of our teachers, the authors of the Shulchan Aruch: It is a mitzvah to seek out a zimun. The greatest among them says "Hodu" and "Ana," and the others answer after him. The greatest may give permission to a minor, and he may join them for a zimun regarding Hallel, even though he did not eat with them. It is the custom that the master of the house recites birkat hamazon on the night of Pesach, as it is said, "He who has a generous eye shall be blessed," and he is the one with the generous eye who said, "All who are hungry, let them come and eat." Thus far his words. And he may honor a distinguished guest. For behold, according to the law of the Gemara, the guest blesses. And the custom is to add in "HaRachaman," "He shall let us inherit the day that is entirely good": "the day that is entirely long, the day when the righteous sit with their crowns on their heads, and enjoy the radiance of the Shekhinah."

And after birkat hamazon, he recites the blessing "borei pri hagafen" and drinks the cup while in a state of hasibah. And if he drank without hasibah, it has been explained in siman 472, see there. And he should not recite a blessing after it, nor should he drink between the third cup and the fourth cup, as is explained in the Yerushalmi, lest he become intoxicated. And it implies that a beverage that is not intoxicating is permitted (Taz). However, from the third cup, even if it is very large, he may drink several times, even though he paused in between, for it is all one cup and one drinking. (See Magen Avraham subsection 1. And I do not know why his intent is necessary for one cup. And it requires further study.) And the acharonim spoke at length regarding what was their custom, that one would leave his house and join for zimun in another house, for this is not proper. Not only for birkat hamazon, but even for Hallel, he should not uproot himself from his place to another place and return to his house. And now we have not heard of this custom. And it is obvious that each person should finish the entire Seder in his house. And after the conclusion of the Seder, if he wished to go to another house in order to answer "Hodu" and "Ana," the permission is in his hand, for all of this is not a requirement. And even an individual can say Hallel by himself, but then he should only say "Hodu" one time and not four times. And so too in the Hallel of Yom Tov and of Rosh Chodesh, and it is simple (see Taz and Magen Avraham and Bach).