It is better to start new groups than to add people to existing groups. Part of the reason is people who come into a group after the third session have missed many foundational truths. If your group is doing the series Creation to Christ or Discovering Spiritual Community, the first few sessions deal with the fundamental concepts of God as creator, God's initial expectations of people and the Fall. These are crucial concepts and people who miss these have missed an important part of how God has revealed Himself to us.
Another reason is that Discovery Groups function best when they have less than 10 people. Every person adds 15 minutes to the group process.
You will also build more leadership if you ask people to start new groups when their friends want to join.
Discovery Groups are so easy to lead that you do not even need to be a Christian to lead them. The leaders are facilitators, not teachers.
When new groups start it is important that a mature Christian meets regularly with the facilitator between sessions to answer the facilitator's questions and to check up on the group dynamic. I sometimes go through each of the questions and ask the facilitator how people responded. The usual sticking points are obedience, meeting needs and telling others what they learned. We then brainstorm ideas on how to help the group members do better in these areas.
Our coaches can help you learn how to encourage your new facilitators.