I live in Zayit which is one of the areas within Efrat. In Zayit there are four main Ashkenazi Shuls. I pay membership fees to one of them. However, I frequently daven with my son at the early morning Shabbat minyan at another Shul. This is not the main minyan at that Shul and there is always plenty of room there. Should I offer to pay membership fees at this other Shul, or should I take the view that since I pay membership fees to one Shul in the area that I live, that I can use any other Shul in the area without paying them as well? Nobody has asked me to pay membership fees from the Shul in which I and my son daven in the early minyan on a Shabbat morning.
It is customary to pay membership dues to one shul which you daven with the most, and identify with. This usually means where you daven on the Chagim and Yamim Noraim, the shul with who’s Rav you have a relationship with, and other similar criteria. Shuls that you frequent in addition to your main shul should receive some donation, but it would seem full membership is not necessary. This may include buying aliyot at some point in the year, sponsoring a kiddush, or just a monetary donation.