Yo! I work within bikeshare, but we're starting to host a lot of group bike rides and such and I've been tasked with thinking about insurance coverage for those rides -- that covers all participants. In looking that up, I learned about the League of American Bicyclist diff insurance options. Maybe something that they offer will be aligned with what you're looking for... or it may be at least a starting point for framing what you might need/want. 

I dropped a couple links below. Give 'em a look. Good luck. -- E