Who (if anybody) requires a minyan for Hallel?

Question

The weekday minyan I go to only says Hallel if there is a minyan present. (I'm talking about for Rosh Chodesh, Chanukah, and chol hamoed; I don't know if their practice differs for chagim but I don't think so.) Today I learned in a comment here that others hold differently:

http://yeshiva.co/ask/?id=3233 says “One May say Hallel alone and at home if necessary.” The site quoted above has a comment “What about Hallel? .. I am fond of a Sefaradi sidur (Bnei Tzion) where it says that Hallel (113-118) is said only with minian ...” http://halachahbyemail.blogspot.co.uk/2009/11/… says “In truth, the requirement is not for a minyan, per se; even having three people present, so that one leads "Hodu" and the others respond, may suffice. (Shulchan Aruch Orach Chaim 422:2; Mishneh Berurah 422:18)”. – Avrohom Yitzchok

So now I'm wondering -- are there other credible opinions that do require a minyan (and my minyan may be following one of them), could there be some other custom at play here, or is this likely to be an error? The minyan is not of Sephardic background. (Past questions of the form "why do we do..." have been met with "that's what we've always done" with nobody seeming to know why.)

Answer 1

It sounds like your minyan is misapplying the opinion of the Rambam.

There are 4 opinions regarding reciting "Half Hallel". (Half Hallel is said on Rosh Chodesh (except Tishrei and Tevet) and the last 6 days of Pesach.) The disagreement is about Half Hallel; all agree to recite Whole Hallel with a bracha even with no minyan.

1. Rambam: No Half Hallel is said without a minyan. With a minyan, Half Hallel is said without a bracha.

2. Rashi (which is followed by Mechaber and Sephardim): Half Hallel is said even with no minyan. A bracha is not recited for Half Hallel

3. Rif (Rabenu Yitzchak Alfasi): a bracha is recited only with a minyan. With no minyan Half Hallel is recited albeit without a bracha

4. The Tur (followed by Rema and most Ashkenazim): say Half Hallel with a bracha even with no minyan.

(Rema concludes that to satisfy the opinion of Rif, one should be careful to recite Half Hallel with a minyan.)

                  Minyan    | No Minyan
         Rambam | no bracha | don't say 
         Rashi  | no bracha | no bracha
         Rif    | bracha    | no bracha
         Tur    | bracha    | bracha

"Whole Hallel" is said with a bracha under all circumstances. On Chanukah, when Whole Hallel is said, it is recited with a bracha even without a minyan contrary to the synagogue referenced in the question. According to Rif and Rambam, when saying Half Hallel no bracha is said even with a minyan; contrary to synagogue referenced in the question.

Source: Shulchan Aruch 422,2 with Be'er Hagola and Mishne Berurah, Tur ibid., and Rambam (Hilchos Chanukah 3:7)