In the diagram above, the basis for the group is formed around a topic of interest or season of life. The 'ungrouped' audience in the weekend service is the market for these new groups. Notice the circles that represent the people. Not much overlap! Only the leader and the assistant leaders (if the he or she has them... a rarity) know one another. The only common thing all the circles overlap is the meeting time and place. Formation is based upon an interest they might have in a topic or their current place in life (new parent, college, career, empty nesters, and so forth), even though it might be called part of a 'spiritual' formation process. When groups are launched in this way, the church has created a new endoskeleton for the leader and unconnected members in hopes they will relate to one another and discover a spiritual purpose together and do more than just meet together.
Classic examples of endoskeleton small group formation would be North Point (Andy Stanley) or Saddleback (Rick Warren) and now Lakewood (Joel Osteen). Each uses their big weekend events as the primary way to pull in unchurched people and then they urge them to join a group that fits an interest or season in life.
The diagram below illustrates a much healthier method of group formation:
The differences between the two approaches are numerous, both in the formational approach, the motivational direction of the basic group and its leadership, and especially the outcome. The group's central focus is not a meeting on a topic or stage of life, it's the everlasting Christ! The leader and his or her core team launch the group because they want to share Christ with their unchurched friends (consisting of existing relationships). As they reach out to unchurched people they know, they overlap their lives with them and share Christ's love in the form of fellowship and servanthood (giving and receiving help from the unchurched friends). The actual small group meeting exists, but it is designed as an exoskeleton for the relational centeredness on Christ and his purposes for the lives of those who launch the group.
Classic examples of exoskeleton small group formation would be Celebration Church (Dennis Watson, New Orleans), Bethany World Prayer Center (Larry Stockstill, Baton Rouge), and a host of cell-based churches around the country with names you might not know.