For a group to be successful it must convey and store certain knowledge of value to future prospective members. Sharing of this tradition, practice, know-how, and derived benefit contributes to the groups growth over time.
Design your community by sharing around a specific topic. Create libraries of supporting materials, discussion groups about particular aspects, and subgroups that orient themselves around topical elements.
The core of the community is its theme. This theme is defined by a key mission and objective. All other actions of the community (subgroups, communications, activities) support specific subtopics around the whole-- focus improves durability.
In a customer community, theme could represent a specific result desired or emotional attitude associated with using a sponsor product. A community is only as strong as the sum of it's parts. Each component must focus on the overall mission and desires of members.