I don't think there are so many of them that you're going to be able to pick the places.

Sacramento Wild Edible Plant Foraging Society might be something to look into.

Wild Food Adventures indicates they are having a workshop in May in Forestville, CA area (Russian River area).

More options if you're willing to go to Oregon.

Blue Wind Botanicals has classes in Oakland teaches regular foraging classes in public parks and gardens in the Bay Area.

Christopher Nyerges has taken thousands of people on Wild Food Outings in the Los Angeles area.

Kevin Feinstein offers some plant walks/workshops in the S.F. Bay area.

Wild Food Walks are held in parks in SF and the East Bay, each lasts about two hours, and covers the wild edibles that are abundant in the area (~$30).

SF — February 5 Saturday, 2-4 pm: Buy Tickets
East Bay — February 12 Saturday 2-4 pm: Buy Tickets
SF — February 19 Saturday 2-4 pm: Buy Tickets
SF — February 27 Sunday, 2-4 pm: Buy Tickets