D-groups, by definition are "open" groups. An open group means that you are open to new members attending, and actively recruiting new members. One of the functions of an open group is to plan on growth, and birth.
Growth, simply means new members. Birth means that you are also looking ahead to the day when your group becomes too large, and you need to "birth" a new group out of the one that you've been meeting with. Your one group, would then become two.
God has always been faithful in meeting with His people as they commit to meet together. Acts 2:42-47.
For the LORD gives wisdom; from His mouth come knowledge and understanding.