What is the meaning of "v'Im Yim'at ha'Bayis mi'Hyos mi'Seh..."?
Rashi: #1: If the household is too small 1 to finish eating the lamb at night, it must join with another household, lest some become Nosar, which must be burned.
Rashi #2, from Pesachim 89a: The Pasuk can be expounded to mean that, if one wants to lessen the numbers in the group (by withdrawing), he must do so whilst it is a Seh (alive), but not after Shechitah. 2
Moshav Zekenim: If one was Makdish his Pesach, and has no money for needs of Pesach or essentials, he may sell part to a neighbor. Even if the neighbor's coins were Hukdash for Pesach, and the Seh is Hekdesh, the coins become Chulin, for Yisrael are Mekadesh their Pesachim and coins for Pesachim with this intent.
Targum Yonasan: The Korban Pesach requires at least ten people to eat it as a group. This seems to follow the opinion of Rebbi Yehudah in Pesachim (91a), who rules that the Pesach cannot be slaughtered for an individual (See Na'ar Yonasan).
Moshav Zekenim: Another opinion there permits withdrawing even after Shechitah, but before Zerikas Dam.
What does the Torah mean when it says "Ish l'Fi Ochlo ... "?
See Ba'al ha'Turim.
Moshav Zekenim citing R. Yeshayah: Even though eating it is not Me'akev, if he is not proper to eat from it, it is Me'akev. How can all of Yisrael be Yotzei on one Pesach (Kidushin 41b)? It is not proper for everyone to eat a k'Zayis!
The Korban Pesach is the only Korban which is brought principally to eat.
What is the meaning of "Tachosu"?
Rashi: It means "You shall be counted (on a Korban Pesach, i.e. own a part)."
Rashi #2 (Pesachim 61a) and Targum Yonasan: "You shall slaughter!" 1
See Peirush Yonasan.
What do we learn from "b'Michsas"?
Rashi: Make a calculation [of how many people will consume it].
Moshav Zekenim (from Pesachim 61a): Only one who was counted on (partially owned) a Pesach may eat from it. 1
Moshav Zekenim: In the Hagadah, we invite [at night] one who needs to eat the Pesach (even if he was not counted on it when it was offered during the day)! Perhaps Shechitah was with intent for people who will want to eat from it later. However, this is unlike Pesachim 89a (which implies that this is only if Yesh Bereirah. We hold that Ein Bereirah for Torah laws - PF).