Parashas Bo · Shemos 12:4 · TT note 2
Torah Temimah on Shemos 12:4, note 2 (on מִהְיוֹת מִשֶּׂה) points to Pesachim 89a.
Sefaria — Exodus 12:4
Sefaria — Torah Temimah on Exodus 12:4
Sefaria — Pesachim 89a
E-Daf — Pesachim 89a
English: One may always enroll on it until there is a kezayis for each person. They may enroll and withdraw their hands from it until it is slaughtered. Rabbi Shimon says: until the blood is dashed. … The Rabbis hold “from being of a lamb” means from the life of the lamb; Rabbi Shimon holds from the existence [as an offering] of the lamb.
Derash of מִהְיוֹת מִשֶּׂה as the deadline for joining or leaving the group.
Enrollment and withdrawal from the Pesach group are permitted until slaughter (or, per R’ Shimon, until the blood is dashed).