Does meggilah require a Minyan And why and how far must one go to get one?(there is a great story about the Gra with this one)
I just found this answer:
The mitzvah of reading the Megillah applies to both men and women. Optimally, the reading should be done in the presence of a minyan (a quorum of ten men), in the synagogue. Even if one has a minyan in his home, it is still preferable to go to the synagogue for the Megillah reading since, as the verse says (Proverbs, 14:28):"The King's Glory is [manifested] in the multitude.” Since one of the reasons that we read the Megillah is to publicize the miracles of Purim, this is best accomplished when it is read publicly in the synagogue.
I think, though, that it is widely held that if one is stuck without a Minyan, one can - and should - still read/hear the Megillah without a Minyan. It is widely practiced that people go around to shut-ins and hospitals to read Megillah to individuals. I don't see why this would be the case if it did not fulfill the Mitzvah.
The previous answer only applies to minyan being read bezmanah (at its proper time). The first mishna in masechet megillah outlines cases where one can read the megillah before the 14th of Adar. Most of these cases do not apply today, but if they did, one would be required to hear with a minyan (see Rambam Megillah 1:7 based on Bavli Megillah 5a).
Interestingly some modern poskim worry that when reading megillah on friday of a purim meshulash in Jerusalem, it may be considered shelo bezmanah (not at its proper time) and would thus require a minyan midina (according to the basic law). I have heard that Rav Aharon Lichtenstein was careful about this when he lived in Yerushalayim.
EDIT: The Mishna Berura in OC 690 sk 66 concurrs that one who reads alone in Jerusalem on Friday the 14th of Adar should read without a blessing.