Right, what you need may not so much be liability insurance, as much as lawsuit insurance. Just getting the case thrown out of court can be expensive.
The most basic issue is that engaging in this activity can potentially subject you to the court system, no matter what insurance you have, no matter what any body signed, no matter how perfect your bike is, no matter how much they paid you.
This is a "cost of doing business", and whether that cost is worth the revenue you're getting is question you'll need to ask yourself (and your lawyer).
I mean, minimally, don't listen to yahoo's like us on the inter webs, take the agreements you signed with Rider Share, take the agreements that the renters sign, and head over to your friendly, family lawyer and say "what kind of exposure do I have here", and see what they say. Ideally they'll say "I haven't the slightest idea" and refer you to a more specialized attorney that can answer it. If it's something that interesting to you, it should be worth a couple hundred bucks to get clarified by a CA attorney.
Ideally, Rider Share has "thought of all of this", but who knows? It is in their interest to protect YOU, as a provider, since, without you, they have no business. But, their first prerogative is to protect themselves.
So, you know, go get it checked out.
..and let us know what you find out!