The Torah commands: "The entire congregation of Israel shall offer it" (Exodus 12:47). The Mekhilta asks why this verse is necessary at all, given that the Torah already instructed each family to take a lamb "for your families" (Exodus 12:21). The answer reveals a significant difference between the original Passover in Egypt and every Passover that followed.
In Egypt, the Pesach (Passover) was organized strictly by family units. Each household selected its own lamb, slaughtered it, and ate it together behind doors marked with blood. The family was the basic unit of that first, desperate night of redemption.
Without the verse about "the entire congregation," the rabbis would have assumed this family-only rule persisted for all future generations. Every Passover thereafter would require eating the sacrifice exclusively with blood relatives — no friends, no neighbors, no mixed groups.
The phrase "the entire congregation of Israel shall offer it" breaks that assumption wide open. For future generations, the Pesach may be eaten "in all kinds of groups" — not just family groups. Friends can form a group. A teacher and students can form a group. Neighbors can band together. The only requirement is that they constitute a recognized group (chaburah) that registered for a specific lamb.
The Mekhilta is tracing an evolution in the nature of Passover. The first Pesach was intimate, household-based, born of emergency. Every Pesach after that was communal, expansive, and celebratory. The circle of fellowship widened from the family to the entire congregation of Israel.