When you join the national organization you become part of a regional group. The annual dues are $40, but the Roundel Magazine by itself is worth the cost. The club has driving schools, clean car events, as well as regular meetings. There is also a big annual "Oktoberfest" which is usually in August. Last year it was in Pasadena - the first time I was aware of that it was on the west coast. Unfortunately, I was not able to attend.
They do have Special Interest Groups, including a M Registry, but I don't see a group specific to the E-39 M5.
BMW CCA #120694