Gemara

(Rav Nachman): If there are three houses, between two Chatzeros, and both Chatzeros put their Eruvin in the middle house, none of the three houses needs to be Me'arev:

The outer houses need not be Me'arev, for each is a Beis Sha'ar to the Chatzer. The middle house need not be Me'arev, for the house in which the Eruv is placed need not contribute to the Eruv.

72b (Mishnah): If brothers eat at their father's table and sleep in their own houses, each of them must be Me'arev.

This is when they put their Eruv in another house [in the Chatzer]. If the rest of the Chatzer brought their Eruv to the father's house, or if no one else lives in the Chatzer, they need not be Me'arev.

Question: What is one's place of Dirah?

Answer #1 (Rav): It is where he eats.

Answer #2 (Shmuel): It is where he sleeps.

Question (Beraisa): If shepherds normally sleep in the city, they are like city dwellers [regarding Shevisah];

If they normally sleep in the field, they have 2000 Amos in every direction [from there].

Answer: There, without any doubt if we would bring their food to the field, they would prefer to eat there.

(Mishnah): If brothers eat at their father's table and sleep in their own houses, each of them must be Me'arev.

Suggestion: This shows that it depends on where one sleeps!

Rejection (Rav Yosef citing Rav Yehudah): The case is, they receive Pras (provisions) from their father.

Rishonim

Rambam (Hilchos Eruvin 4:6): If brothers have their own houses, and do not rely on their father's table and sleep in their own houses, and similarly wives or slaves who do not always eat at the table of their husbands or masters, but they eat on his table in reward for their work or for gratitude on certain days, like one who eats with his friend for a week or month, if there are no other residents in the Chatzer, they need not be Me'arev. If they made one Eruv with another Chatzer for all of them, if the Eruv came to them, they need not give bread. If there were other residents in the Chatzer, each must give bread, like other residents of the Chatzer, because they do not always rely on one table.

Rebuttal (Ra'avad): He learns from the Mishnah "if brothers eat at their father's table and sleep in their own houses, each of them must be Me'arev." The Gemara established that this is when they receive Pras. The Rambam's Perush is wrong regarding women and slaves. Rather, the father, husband or master gives to them their provisions, and they eat in their houses. Since they are fed from one place, and eat in their houses, they are partially divided and partially united.

Rosh (7:8): A wife can be Mezakeh an Eruv when she has a house in the Chatzer, since she can acquire [a share] for herself. What is the case? If she receives Pras from her husband, she need not acquire for herself. If she does not receive Pras from her husband, in any case she can acquire for others, since her husband does not acquire what she acquires! I answer that her husband gives her money for food. This is not considered receiving Pras regarding Eruv when he does not give to her the food from his house... We conclude like R. Yehudah ben Bava, who says that a wife who receives Pras must be Me'arev.

Poskim

Shulchan Aruch (OC 370:5): If one eats and sleeps in different places, where he eats is primary. He forbids there.

Mishnah Berurah (42): This means that he sleeps in another Chatzer.

Beis Yosef (DH Mi): All the Poskim rule like Rav, who says that where one eats determines his Dirah.

Kaf ha'Chayim (58): If there are other residents where he sleeps, and they were not Me'arev, just like they forbid each other, they forbid also him.

Bi'ur Halachah (DH v'Yashan): The Pri Megadim says that if Talmidim eat in a host's house and sleep in their Rebbi's house, their Dirah depends on where they sleep, for the same reason we say so about shepherds, i.e. surely they would prefer to eat in the field if we would bring their food there.

Shulchan Aruch (ibid.): If brothers eat in their father's house, and sleep in their own houses, they do not forbid.

Gra (22, 25): The Gemara tried to prove that it depends on where they sleep [but rejected this, and said they receive Pras. This connotes that] if they truly eat on their father's table, they never need to be Me'arev.

Mishnah Berurah (43): They do not forbid the place where they sleep, if they were not Me'arev, nor the place where they eat, for they are drawn after their father and they are Yotzei through his Eruv.

Shulchan Aruch (ibid.): If he feeds them and they eat in their houses, they forbid.

Beis Yosef (DH u'Mah and DH u'Mashma): We tried to prove from the Mishnah that the place of sleeping determines Dirah. The rejection was that they receive Pras from their father. Rashi explains that their father pays for their food, but they eat in their own houses. It seems that it does not matter whether they receive food, or money for food. Tosfos and the Rosh wanted to distinguish between them, but this was only in the Hava Amina. In the conclusion, there is no distinction. The Rambam connotes that he explains that they receive Pras and do not always rely on their father's table, just sometimes they eat on it for a week or month as a reward for work that they do for him, or for gratitude.

Magen Avraham (10): This is even if they receive money to buy food. The Gemara connotes that if shepherds sleep in the field, even if they eat in the city, they do not forbid, for surely they would prefer to eat in the field if we would bring their food there. This requires investigation. See 409:7. (If Talmidim eat in a host's house and sleep in their Rebbi's house, we measure their Techum from where they sleep, for surely they would prefer to eat there if their food was there.)

Kaf ha'Chayim (61): The Rashba (Avodas ha'Kodesh) and Ritva explicitly say so [about shepherds].

Mishnah Berurah (45): They forbid others, and also themselves, when they were not Me'arev, even if they live in the same Chatzer with their father, since they do not truly eat on his table.

Shulchan Aruch (ibid.): Some say that this is only when the rest of the Chatzer put their Eruv elsewhere, but if they put it in one of these houses, or there are no other residents in the Chatzer, they need not be Me'arev.

Beis Yosef (DH u'Mah she'Chasav Rabeinu): Rashi explains that when they send their Eruv to another house, since they needed to be Me'arev, we say that just like the other residents forbid, also they forbid. If the Eruv for the entire Chatzer comes to their father's house, they did not need to be Me'arev, for the house in which the Eruv is placed need not give bread. They are all drawn after it. If there are no other residents in the Chatzer to force them to be Me'arev, they need not be Me'arev, for they are like individuals

Mishnah Berurah (46): When there are no other residents in the Chatzer, they do not obligate each other to be Me'arev. Since they receive Pras from their father, this joins them to be like one person. Likewise, when the Bnei Chatzer put the Eruv with one of the brothers, since he is exempt from giving his share to the Eruv, since the Eruv is in his house, also the other brothers are exempt for the above reason.

Mishnah Berurah (47): 'One of these houses' means the father's house, or one of the brothers' houses.

Bi'ur Halachah (DH v'Hani): The Shulchan Aruch did not clarify whether or not this is only when the brothers are in the same Chatzer with their father, or even if they are in another Chatzer. The Ritva was unsure about this. Rashi connotes that it is only if they are in one Chatzer. R. Yerucham connotes that it is even if they are in another Chatzer. This requires investigation.