Try Marin Cyclists (find them online at http://www.marincyclists.com) for group training rides. Marin Century in August and Chico Century in late April are my favorite organized NorCal rides as well as Levi's Gran Fondo in Sonoma County around October.
What kind of rides are you looking for, group training rides, centuries, races, new routes?
Tour de Cure Napa, Lodi Sunrise Century, Delta Century, Wine Country and Monticello centuries in May, Tahoe ride and Sierra Century in June, Death Ride in July, Marin and Napa Century in August.
Benicia Bike Club, Solano Family Cycling Club, Monticello Cycling, and Davis Bike Club have regular training rides in Solano, Napa, Contra Costa, and Yolo counties on almost any day of the week.