Organizations
These are clubs I am a member of, or at least connected with. That means they're probably in the SF Bay Area or have a presence in the SF Bay Area.
Costume Clubs
Silicon Web is my International Costumers' Guild chapter. It's an internet-based group that communicates primarily through an email list, with members scattered across North America.
The Greater Bay Area Costumers' Guild is an SF Bay Area costume club. It tends to be a bit skewed towards the historic costume side of things, but it's not an exclusively historic costume group.
Wearable Art Clubs
The Peninsula Wearable Arts Guild is one of the oldest art-clothing clubs in the country, and it's a really fun group. There's a meeting and workshop the second Saturday of every month. Members bring a wildly diverse collection of skills to the table, and if there's a sewing or embellishment technique you want to learn, you're going to find someone who can help you.
Several PenWAG members are also involved in the Surface Design Association. I'm not a member, but I do follow their blog. Their Northern California group is pretty active.
Event Organizers
The Period Events & Entertainments Re-Creation Society is a vintage dance club that schedules monthly costumed dance events. You might think from the name and description this is a purely historical costume venue. It's not. PEERS does fantasy and science fiction themed events too.
The Bay Area English Regency Society is best known for regency dances. They produce other events, sometimes with a bit of a twist.