Disciples are rarely made in rows. From an environmental angle, a disciple is far more likely to made in a circle. After all, becoming a disciple has far less to do with digesting information (like in a class) and far more to do with spending time with those who are becoming like Him.
Discipleship is life on life. Even the leader needs to open up and be apart of the group. Leading a D-Group is less about being an instructor and more about being a mentor. You are guiding people along the path you have already been on, while letting them into your life as a trusted friend.