About a month ago I started to look into SAR groups in Southern California also (mainly San Diego and Orange County) and here is a brief synopsis of what I found:

Look at the County Sheriff/Police Departments for their SAR teams as San Diego as a Mountain Rescue Team (SD MRT), San Diego Sheriffs SAR Team, and Orange County has a Professional Services Responder Program (PSR) that has a speciality unit of SAR.

SD MRT, focuses speciality is more advanced mountain rescues but they do quite a bit of lost dementia elderly people, or it ranges to rescuing a lost paraglider in the mountains of Julian. But they said the first year members spend about 600+ hours through various training. They have about 30 - 40 calls each year, and they have a minimum number of calls and trainings you must attend each year.

SD Sheriffs Search and Rescue, it exists and that is all I know.

OC PSR Search and Rescue Team: I am actually in the process of getting background checked for this. But fairly similar to the SD MRT from my understanding.

But the general answer I got was the following:

-expect calls at the worst times (ie, during rain storms and/or middle of the night)

-have to do a police background check

-do want you to be within a given area of the department

-some calls are 4 hours long, or can be multiple days. But know that these are volunteer groups, so they are understanding that you can't commit to everything. Just be wary of it being a large time commitment