The very first thing you should do is protect your right to homeschool. These people can help you with that:
http://www.hslda.org/
They also have a whole site devoted to finding homeschooling support and umbrella organizations in your local area:
http://www.hslda.org/orgs/default.asp?State=CA
If none of these groups are secular you could always run an ad or put a postcard with contact information up at your local library. I founded a GREAT, secular military family group here in Omaha that way. I simply wrote, "Secular, eclectic homeschoolers looking for other homeschooling families." with a phone number. I had five calls the first week. There are thirty families in our group now. :-)
Yahoo groups is a great resource too, believe it or not. I typed in "secular homeschools" and was bombarded with 160 groups! Granted, not all of them were in California, but it might be worth looking into.