Welcome them, give them pointers on getting started, suggest topics where they might participate, and tell them about any interesting upcoming events. Make the discussion as permeable as possible. This means creating regular breaks in the discussion, through summaries or open questions, where a newcomer might sense a space for themselves. Provide a place where newcomers can experiment with the software. While the community owner can largely manage the first three steps, this final step is wholly dependent on the entire community.
People return to places where they find a group of people to talk to.
If you do not have such a place, you will not be able to retain members. No effort on your part, no matter how great, will create a regular where there is no community. This means that creating a community involves, in large part, knowing when and how to let go and allow the community to take ownership. After a certain point, efficiency runs contrary to community. Find things you DON T do well, and let your members know that you need help in those areas.