does your area have a more locally centered Bulletin board or forum? I know we have one in my area and people have used it to find trail riding buddies.
Simply networking with other riders that you know can often lead to meeting other people that are interested in trail riding as well. If you ride with a trainer sometimes they can help network you with others as well. I've met several great riding friends through trainers I've worked with.
You could try posting an ad at local feed stores, also the horsetrader magazine has free online ads, plus so cal equine and so cal equest have online ads. There is also an ETI group in OC, check the ETI main page and look for the corral of that area, they might have a local newsletter.
I'm way north of you in Ventura County, or I would ride with ya!