Not sure about a the incorporation details but my club (in BC, Canada, so YMMV) is a member of Cycling BC. Each member has to be a member of the club and a member of cycling BC (we get sort of a group discount), which provides for insurance. We require people who ride with us to be a member so that liability and insurance issues are taken care of. As the rides are put up by the club, the club is responsible for them.

My guess is that we're also incorporated, which clarifies legal responsibilities as well...

Not sure about the US, but incorporation is not that big of a deal, if a little costly (around $200 here). But then you need to have a board and an annual meeting of members.

I'm pretty sure you can get started informally, but as you grow (and seek sponsors?) being incorporated will help.

Good luck!!!