Eruvin, Chapter 6, Mishnah 6. The Mishnah deals with five separate groups that spent Shabbos together in a single traklin - a large house, what we would call a hall or a palace - where the traklin is divided into five separate areas by partitions that do not reach the ceiling, with each group staying in its own divided area.
The question under discussion:
Is each group considered a separate group, so that each one must contribute its share to the eruv chatzeiros, or are they all considered one large family?
The dispute between Beis Shammai and Beis Hillel:
Beis Shammai say: "eruv le'chol chavurah va'chavurah" - each of the groups must participate in the eruv on its own.
Beis Hillel say: "eruv echad le'chulan" - it is enough for one of them to participate on behalf of them all in the eruv of the courtyard. That is, all the groups together give only one contribution to the eruv between the houses of the courtyard, since they all represent a single house.
The case in which Beis Hillel agree:
However, "u'modim, she'im hayah miktzatan sheruyin ba'chadarim o ba'aliyos - she'hen tzerichin eruv le'chol chavurah va'chavurah" - when some of them are staying in separate rooms or in upper stories. The Gemara explains that this refers to partitions between the rooms that rise all the way to the ceiling, so that these are entirely separate rooms. In such a case participation is required from each and every group in the eruv chatzeiros, because they are considered separate houses in the courtyard.
In summary: Where five groups spent Shabbos in one traklin and the partitions between them do not reach the ceiling, there is a dispute: according to Beis Shammai each group must make an eruv of its own, while according to Beis Hillel one eruv suffices for them all, since they are considered a single house. But when the partitions do reach the ceiling and the rooms are completely separate, Beis Hillel agree that each and every group needs an eruv of its own, since their status is that of separate houses in the courtyard.