Gemara

(R. Yonason): If a Beis Din of 100 Chachamim gathered to give a ruling, they are liable only if all 100 agreed. We learn from "v'Im Kol Adas Yisrael Yishgu" - all the judges must err.

Beitzah 5a (Rav Yosef): Anything forbidden through a Minyan (vote of Chachamim) requires another Minyan to permit it. We learn from "Shuvu Lachem l'Ohaleichem" and "bi'Mshoch ha'Yovel Hemah Ya'alu v'Har."

Rishonim

Tosfos (Beitzah 5a DH l'Davar): Some learn from here that a Cherem until Pesach or a fixed time needs another Minyan to permit it. Even though Hash-m forbade Bi'ah (relations) for three days leading up to Matan Torah, He needed to permit. This is wrong. The verse of three days teaches to prepare themselves for what will be in three days. It does not say "be ready (refrain from Bi'ah) three days, rather, "l'Shloshes Yamim" (for what will be in three days). The Isur of Bi'ah was permanent, therefore, it needed a Heter.

Rosh (Beitzah 1:3): Anything forbidden through a Minyan requires another Minyan to permit it. This is when something occurred that makes it known that the decree is Batel, e.g. it is known why it was decreed, and the reason does not apply. It is proper that the decree be Batel automatically. However, it is not Batel until Chachamim vote to permit it. However, if a time was set for the decree, when the time passes the decree ends, and no Minyan is needed.

Poskim

Shulchan Aruch (EH 1:10): R. Gershom made a Cherem that a married man may not marry a second wife.

Rema: If his first wife cannot be divorced, e.g. she went insane, or the Halachah is that he must divorce her but she does not want to take a Get from him, we can be lenient to permit him to marry another, and all the more so if she is an Arusah and she does not want to have Nisu'in or be divorced.

Shulchan Aruch (ibid): The Cherem was only until the end of the fifth millennia (from creation, i.e. 1240 C.E.).

Rema: In any case, in all our lands the enactment and the custom is persists, and one may not marry two women. We use Cherem and Niduy to force one who marries two women to divorce one of them. Some say that now we do not force, for the fifth millennia already ended. This is not the custom.

Darchei Moshe (9): Nowadays, we do not need 100 people to permit the Cherem anywhere, for the time of the decree passed. Tosfos and the Rosh say that no Heter is needed at all. The decree is Batel. Now it is merely a custom to be stringent, and one may not permit it in front of others. Maharik says that the Rashba permitted when letter of the law one should marry another or divorce his first wife. Presumably R. Gershom did not decree about this. All the more so we may be lenient, since we heard that the enactment was only until the year 5000. One who relies on the lenient opinions does not lose. He did not decree due to an Isur mid'Oraisa. It was a mere enactment. We follow the lenient opinion in mid'Rabanan laws, and all the more so in a mere enactment.

Bach (Sof ha'Siman): Acharonim cite a tradition from the Beis Din of R. Gershom that 100 Rabanan can permit the Cherem if there is a proper reason. We need 100 lest it be light in the eyes of later generations to marry a second wife. This is the custom in our times when a wife goes insane. It is common to get 100 Rabanan to permit.

Chasam Sofer (EH 3): R. Gershom made two enactments. He forbade to divorce Bal Korchah. This was a fence for Torah, due to Perutzim who disgraced proper Bnos Yisrael. He did not fix a time limit for this. It spread throughout all of Yisrael, and he did not stipulate to permit it through 100 Rabanan. However, almost all Ge'onim agreed that for the sake of a Mitzvah mid'Oraisa, e.g. Bitul of Peru u'Rvu or Yibum, he never enacted. The second enactment, not to marry two women, was not a fence for Torah. It was merely to avoid feuds. It did not spread throughout all of Yisrael, and it was only until the years 5000. However, the Yam Shel Shlomo (Yevamos 6:42) says that since there was no Minyan to permit it, the Cherem still applies. However, R. Gershom stipulated that if Chachamim of the generation see a reason to permit, 100 Rabanan can permit it. Since it has this leniency, it was not automatically permitted for the sake of Mitzvos. When she is blameless, e.g. she is barren or sees blood due to relations, he cannot divorce her Bal Korchah until toiling to get 100 Rabanan to agree. When she is to blame, e.g. she acts immodestly, he divorces her without 100 Rabanan.

Meshiv Davar (4:4): Rabbeinu Meir mi'Rottenberg says that 100 Rabanan from three lands from three congregations may permit R. Gershom's Cherem not to marry two women. They must see a clear reason to permit. The husband must have the money or securities for the Kesuvah ready. We cannot say that the Heter is for a wife who did not give birth (in 10 years), for we force him to divorce her or marry another woman! Teshuvas Maharshal (35:5) says that Avi ha'Ezri did not agree to permit the Cherem even if Bitul Peru u'Rvu would result. It is better to lose one soul (he will not have children) than to cause ruin in the future. He did not mention the Heter of 100 Rabanan. Either he did not hear of this Heter, or it was impossible to get 100 Rabanan in his time. Maharshal says that this was only to divorce Bal Korchah (against her will), but not for marrying a second wife. We hold like Maharam and those with him, that there was an enactment to permit through 100 Rabanan when needed. The Rema holds that there is no need for 100 Rabanan. He never mentioned this (in his Hagahos Shulchan Aruch). There is an absolute Isur to transgress a custom that many uphold. The Chasam Sofer (4) says that Lo Yachel Devaro applies. That is only if one himself fulfilled a certain Mitzvah, for it becomes like a Neder, but not for a custom of Rabim. In any case, one may not transgress the custom, due to "v'Al Titosh Toras Imecha." Many Poskim say that if one transgresses this, we put him in Cherem. In any case the Rema holds that it is no more severe than an individual's vow, which Beis Din can permit. Likewise, a custom thatinitially required 100 Rabanan to permit it, nowadays three can permit it. Later Ge'onim, i.e. the Bach and his camp, did not agree. We hold that the enactment keeps its initial strength; 100 Rabanan are needed to permit it. They must be from three congregations from three lands, so it will be the most important Hora'ah in Yisrael, i.e. a Great Sanhedrin of 100, like it says in Horayos. The Great Sanhedrin was of three lands in Eretz Yisrael - Yehudah, Galil and Ever ha'Yarden. It is best if they all gather together, unlike the Chasam Sofer, who said that they must be scattered, to make the Heter harder. That is wrong. This is not an Aveirah, which we must make hard for people! Also, why should it be harder for an Oni, who must go by himself, than for an Ashir, who can send Shluchim?!

Igros Moshe (EH 1:2): It is unreasonable to be more stringent about divorcing Bal Korchah for the sake of a Mitzvah, due to the leniency that 100 Rabanan can permit it. One Rav qualified to give Hora'ah can rule about Isurim of greater severity. Why do we require 100 Rabanan! We can give two reasons. 1) The Isur applies in every case, even if she went insane. However, 100 Rabanan in any generation have power to permit it, even if they are small in Chachmah and Minyan than those who forbade, if they see a great need. We need 100 to permit, for it is not a Hora'ah to clarify the Halachah. Rather, it is permitting the Cherem. He specified that 100 are needed to permit it. A Tanai enables a small Beis Din to permit what a greater Beis Din forbade. 2) Even if he never forbade for a woman who went insane - it is unreasonable to forbid and stipulate that Rabanan permit it - this is a different enactment for such cases to require a Hora'ah of 100 Rabanan, for often great toil and expense induce him to agree to bear such a wife. Many lunatics are slightly suitable for marriage if the spouse is willing to toil and bear (difficulty). Whenever we permit because one cannot live with such a woman, it was enacted to require 100. The toil and expense might make him reconsider his desire to marry another. Wearying people can change their resolve, e.g. to make a Sotah admit her guilt or witnesses retract from intent to testify falsely (Sotah 8a). Here, the toil can help him decide to bear her, or wait longer in case she will be cured. Based on this, we do not need 100 Rabanan if she is forbidden to him. We force him to divorce her. We should not make it hard for him. Perhaps he will decide to keep her! Even though he may divorce her Bal Korchah, he needs a Heter to marry a second wife when it is impossible to divorce her. Based on the first reason, perhaps the enactment was Stam, in every case. It is more reasonable that there is no enactment about such women. The same applies to an apostate, who is forbidden to him. However, if a woman is barren after 10 years, perhaps 100 Rabanan are needed. This is not because she is blameless, like Pischei Teshuvah (16) brings from Shev Yakov. Rather, sometimes it is improper to divorce, and each case must be investigated. Also, based on the second reason, perhaps he will retract. If a woman sees blood due to Bi'ah, we do not require 100 Rabanan, for it is rare that she be cured, and one may not keep her.

Achi'ezer (1:10:2): Requiring 100 Rabanan creates publicity, so siblings will not come to marry each other. Beis Hillel says that nowadays, R. Gershom's Cherem not to marry two women does not apply. We must say that even if she went insane, we do not need 100 Rabanan to permit it. Alternatively, marrying the second wife forbids his first insane wife, so the enactment does not apply.