Here is a quote from the commentary section in my Nusach Ashkenaz Artscroll Siddur:
Tefillin on Chol HaMoed
There are three different customs (all halachically valid) regarding the wearing of tefillin on Chol HaMoed:
a) Tefillin are worn but the blessings usually recited upon donning them are omitted (Taz to O.C 31:2).
b) Tefillin are worn and the blessings recited, but silently (Rama).
c) Tefillin should not be worn (Orach Chaim 31:2 and Vilna Gaon).
And, later down it continues to say:
It is not proper for a congregation to follow contradictory customs. Thus, if one whose custom is not to wear tefillin during Chol HaMoed prays with a tefillin-wearing minyan, he should don tefillin without a blessing. Conversely, if one whose custom is to wear tefillin prays with a non-tefillin-wearing minyan, he should not wear his tefillin while praying but may don them at home before going to the synagogue (M.B).
Note that the common Ashkenazi custom in Yerushalayim follows the minhagim and rulings of the GR''A (The Vilna Gaon).