Most legal forms of organization will require some form of structure with by-laws. A quick Google yields:
Kalamazoo LUG by-laws in PDF format - likely not what we want, as they require dues for privileges such as email archive access, but these can serve as a starting point for discussions.
Liability is a concern raised several times, most recently at SummerSummit2006. Googling provides lots of information. Consultation with legal advice and insurance professionals may be advisable.