Growing and maintaining an open-source community depends essentially on three things:
Getting people interested in contributing
Removing the barriers to entering the project and contributing
Retaining contributors so that they keep contributing
If you can get people interested, then have them actually contribute, and then have them stick around, you have a community. Otherwise, you don't.
If you are just starting a project or need to improve the community of an existing project, you should address these points in reverse order. If you get people interested in a project before you do the later two steps, then people won't be able to enter and won't stick around when they do enter. You won't actually expand your community.
So first, we want to be sure that we can retain both existing and new contributors. Once we've done that, then we want to remove the barriers to entry, so that interested people can actually start contributing. Only then do we start...