Clubs and Groups

Meetup is the world's largest network of local groups. Meetup makes it easy for anyone to organize a local group or find one of the thousands already meeting up face-to-face. More than 2,000 groups get together in local communities each day, each one with the goal of improving themselves or their communities.
www.meetup.com
In fact, there's a meetup designed specifically for empty nesters:
emptynest.meetup.com

Yahoo Groups - Find a group that's already started and join in on any subject or issue.
groups.yahoo.com

Association of Collecting Clubs - If you like collecting, this site provides direct links to the membership applications for many of the Association of Collecting Clubs (ACC) member clubs.
joinaclub.org/Intro/Clubs

Sierra Club - This is a great way to get involved and meet people and take action to save our environment with one of the oldest green groups around.
www.sierraclub.org