Actually, since your doing a form of possible ride sharing you might be subject to the beefy insurance regulations found here
http://leginfo.legislature.ca.gov/faces/billNavClient.xhtml;jsessionid=f4d692baeba37abf7ce3b0b61b6e

If you need that policy, it starts at 50k!
Local transport companies are subject to different regulations but are required to carry by law in CA:
Work comp for driver
General Liability
Vehicle insurance including liability, accident, and possibly physical damage.

I can help for a reduced fee if you have additional questions on this topic as we are also compensated by the insurance companies for the policy placement.