Follow up to this answer.
The Mishnah Berurah refers to the shul's minhag.
While on vacation, I entered a shul carrying my tefillin bag. In the shul, I saw 6 people wearing tefillin, one of them was the rabbi. The other 4 were not wearing tefillin. I ask the rabbi, "What is the shul's minhag?"
The rabbi says, "I'm new here, and until I came, this shul never had a daily minyan. When I came, I tried to establish my rule of wearing tefillin on Hol Hamo'ed, and some people protested and said that they wouldn't come here since they don't wear tefillin. Seeing that I wouldn't have a minyan, I couldn't establish any shul minhag at all, so technically, there is none. I allow people to follow their own personal minhag."
I'm not sure that the rabbi's answer made "halachic" sense, even if it was practical (he was trying to assure a minyan.) But which minhag should have been followed? One's personal minhag (seeing that the shul has none?) The minhag followed by the majority of people in the shul, or the rabbi's minhag, as he is the "leader" of the shul?