At a Pesach Seder, when you get to the parts of Hallel that are done responsively in a congregation, are you supposed to do them responsively? Does the answer depend on how many people are at the Seder?
If there are three people who recite the Hallel together, the two responsive readings (the four verses ending "Ki LeOlam Chasdo" at the beginning of Psalm 118 and the four "Ana Hashem" verses toward the end of that psalm) are recited as in the Shul: the leader recites each of the "Ki LeOlam Chasdo" verses and the others answer "Hodu" [and the next verse quietly; in some communities the leader recites "Hodu" quietly with the others], and the leader recites each of the "Ana Hashem" verses followed by the others.
For all Hallel recitations throughout the year, only men are obligated - at it is indeed preferable to ensure that there are at least three adult men at the Seder to recite the Hallel (Shulchan Aruch 479:1). However, at the Seder, women are also obligated to recite the Hallel; thus, they - and children who have reached the age of chinuch - can be counted at the Seder in the count of the three people for these recitations (Magen Avraham and Taz ibid.).
If there are only one or two obligated persons at the Seder, one should not use the repetitive reading (Mishnah Berurah 479:11).
The way I heard it, they're done responsively if you have three adults (presumably all male?); that way, one can say, "you (plural), gave praise to Hashem", and the other 2+ can respond.