There are almost always opportunities to volunteer. However, joining CAP is a larger commitment than simply saying "I'd like to volunteer to fly." You will need to attend regular meetings and perform non-pilot duties such has becoming an administrative officer or participating in recruiting and fund raising activities. For some squadrons, the budgets are tight, and the ratio of pilots to flights can be high. The complaint I heard from CAP members back in Idaho was that they hardly ever got to fly. This may be less true in other areas.
My suggestion to you would be to contact your local squadron. In your case, this will be a squadron within the California Group 2 (San Francisco Bay). They have senior squadrons located in Palo Alto, Oakland, San Jose and elsewhere (senior squadrons are for adults 18+, whereas cadet squadrons are for children under 21. There are also composite squadrons for both seniors and cadets). Talk to them about their mission and what the experience would be like. If you're still interested, attend some meetings (you'll need to do this before they'll let you apply anyway). Then, after you have a good sense of what will be expected, if you believe in their mission, and you're willing to put in the time required, you should join.
If all you're looking for is a way to build cheap (or free) flight hours as a private pilot, then that is probably the wrong reason to join, and you will likely be disappointed.
*Note from editor: The oddity in the ages of membership for the senior and the cadet groups should be explained. A person may join as a cadet in a squadron up to their eighteenth birthday, however, if they became a cadet before that, they may continue as such until their twenty-first birthday. Anyone joining the Civil Air Patrol after eighteen enters into the senior branch.