Pesachim, Chapter 8, Mishnah 4. This mishnah deals with a person who has a share in the Pesach offering, and with the question of his status when he wishes to register others onto his own portion.
The case before us:
A person registered for the Pesach offering together with two other people, so that a third of the offering is his. Now he wishes to register an additional person onto his portion, and he says: "ani notel et hashelish sheli vecholek oto beini uvein adam acher" - "I am taking my third and dividing it between myself and another person" - and he is permitted to do so.
The ruling of the mishnah:
The rest of the group: the two people who were not partners to the inclusion of this fourth person are entitled to give him his portion.
The registered member and his new partner: they eat from their own portion, while the members of the group eat from theirs.
The members of the group are not required to remain a single group together with the one who registered with them originally, once that partner decides to add an additional person onto his portion.
The reason:
They are permitted to split into two separate groups, because a Pesach offering may be divided between two groups, and the fact that they began as one group is no obstacle to this. Therefore the original registrants are not obligated to be registered together with a person whom they themselves did not register.
In summary: in this mishnah we learned that one who holds a share in the Pesach offering may register others onto his own portion, and the rest of the group are entitled to give him his portion and separate from him - he eats from his own together with the one he registered, and they eat from theirs, since a Pesach offering may be divided between two groups, and a person is not obligated to be registered together with someone he did not register.