The leader has to bring the group together. Most groups form with common interest. By this I mean there should be something which all of the members have in common. If the group is too large, then mitigate it by relieving all members from the group who do not share the same interest. Also, having every member to introduce themselves and state why they have joined the group is an icebreaker. You can also start the first group meeting off with a team or group exercise that would help everyone get comfortable with one another.