If someone wants to get dressed up in a uniform, and march around playing "bang you're dead", it is generally not too difficult to find some kindred spirits to play with.
However, for those who are more interested in the more peaceful aspects of history, it is sometimes a bit harder.
For those who wish to participate as a civilian in local Civil War events, you might want to consider the "Historical Citizens Association"
This is in addition to civilians (mostly female) who attach themselves as an auxiliary to military units (too numerous to list here).
Another great option is to participate as a costumed volunteer at one of the local historic parks. The ones I know about with active costumed docents programs are below:
Los Encinos State Historic Park in Encino
Pio Pico State Historic Park in Whittier
Banning Residence Museum in Wilmington
And of course, let's not forget vintage dancing, of every variety.
While it isn't exactly "peaceful", for those interested in the more violent aspects of the Old West, there is SASS (Single Action Shooting Society), which has a large, mostly female, non-shooting contingent.
My personal favorite tactic however, is to be the ultimate civilian and not do a lot of joining. When something comes up that seems like it might be fun to dress up and attend, I'm there.