Eruvin 72: Where You Lay Your Head Down

A dad of two mishnayot. 1. The case of several groups in a large hall... A dispute whether every group would need to contribute to an eruv in that space or not. When are physical dividers among the groups considered sufficient partitions to divide among the people, and when are they low enough to simply divide the space, but not line up halakhic areas for eruv. Which leads us to speculate about the role of residence, or the absence thereof. 2. The case of brothers who sleep in their homes, and convene at their father's home for meals - do they need an eruv for their entire area, or not? And if they don't, then why is this case here to begin with? The notion that preserving halakhah for cases that are not then at hand is an ongoing orientation by Chazal. What about one who has multiple residences - where does he need to make eruvin?