Everyone is obligated in the mitzva of reading the Megilla: men, women, and converts. One who hears someone else read the Megilla discharges his obligation, as long as the reader is obligated in the mitzva. However, if one hears it from a minor, who is not required to observe mitzvot, he has not fulfilled his obligation (sa 689:1-2).
The main purpose of reading the Megilla is to publicize the miracle and demonstrate that God rules and oversees the world, directing everything for the best. Even the worst troubles eventually turn around for the good. This understanding strengthens people’s faith in God and stimulates them to do more to reveal His name and rectify the world.
Reading the Megilla in public, to broadcast the miracle, is so important that even the Kohanim working in the Temple would delay the daily Tamid offering in the morning in order to hear the Megilla with the congregation; only afterward would they offer the Tamid. Similarly, Torah scholars who are occupied by the study of Torah, even if they can read the Megilla with a minyan in their study hall, should nevertheless interrupt their studies in order to go to a synagogue and hear the Megilla together with the masses (Megilla 3a).
Therefore, the members of a synagogue that usually hosts several minyanim every day should try to gather together on Purim and hear the Megilla with a large number of people. However, one who generally prays in a small synagogue need not change his fixed practice in order to hear the Megilla in a large synagogue, provided that there will be a minyan at the reading he attends (sa 687:2, Ĥayei Adam, mb ad loc. 7, sht ad loc. 8, 10).
Only as a last resort – if one is unable to hear the Megilla read in a minyan – may one fulfill the mitzva in private, with a berakha (sa and Rema 690:18).
The Talmud records a dispute in Megilla 5a: Rav maintains that when Megilat Esther is read in its proper time, even an individual may read it, while R. Asi maintains that a minyan is required even if it is read in its proper time. The Rishonim differ over how to interpret this dispute. According to Rabbeinu Tam, the dispute concerns a le-khatĥila situation, and the halakha follows Rav’s opinion. Therefore, even le-khatĥila, an individual may read the Megilla, in its proper time (though it is clearly preferable to read it with a large group of people). According to Behag, the dispute concerns a be-di’avad situation, and the halakha follows R. Asi’s opinion. Therefore, an individual should not read the Megilla. Those who take Behag’s opinion into consideration maintain that an individual reading the Megilla should not recite a berakha (Mordechai, quoting Rabbeinu Gershom, is in this vein, as is Mahari Weil). Many Rishonim maintain that, le-khatĥila, one should read the Megilla with a minyan, but that this is not absolutely necessary in order to fulfill the mitzva. The reason for this is that either the halakha follows R. Asi’s opinion, though even he would agree that, be-di’avad, an individual may read the Megilla; or the halakha follows Rav’s opinion, and even he would agree that, le-khatĥila, one should read the Megilla with a minyan. Rosh and Raavad follow this position, and it is codified in sa 690:18. The vast majority of Aĥaronim agree that one who reads the Megilla individually should recite the berakha. Orĥot Ĥayim quotes Raavad as saying that if the Megilla was read in front of ten men in a certain place, an individual in that place may read it alone le-khatĥila, as the miracle has already been publicized. Rema 690:18 concurs. mb ad loc. 64 states that some rule stringently and require an individual to search for ten men, le-khatĥila, even in this situation. See below, n. 8, regarding whether a woman may read the Megilla on behalf of other women and whether she may recite the berakha for other women in doing so. Also see n. 17, on the issue of whether a minyan is required for the berakha of “Ha-rav et riveinu.” (According to Ben Ish Ĥai, one recites the berakha even without a minyan, but the prevalent custom is not to recite it. If ten women are present, one should recite the berakha.)
According to Rif, Rashba, and Ramban, one should not read the Megilla without a minyan when it is not the proper time for reading it (11-13 Adar). Rashi and Ha-ma’or, however, maintain that one may read the Megilla and recite the berakha, be-di’avad, without a minyan. In practice, sht 690:61 states that if one reads the Megilla without a minyan when it is not the proper time, one should not recite the berakha.