It's not really a playgroup, but Communication Works is a company in Oakland that offers Social Integration classes for autistic spectrum kids of all ages. Kids are put into a class of 2-4 kids with a therapist. I believe that all of the therapists are trained by Michelle Garcia Winner down in San Jose. The kids in each group are hand selected for compatability. It's been a fabulous program for our 4 year old.
ALso, have you heard of Bayhill High School in Oakland? It'a a private high school for students with language based learning disorders. It sounds amazing.
Hope that's helpful.