The groups functionality on LinkedIn sucks. It's also unclear what is "acceptable" practice in terms of communicating with members of the group.
The OnStartups.com group for startup fanatics (http://www.linkedin.com/e/gis/2877/30950504D0B7) has 7,000+ members, but there's so much that could be done with it if LinkedIn had some simple community features -- or, finally got around to releasing an API for app developers.