Been trying to build up the dev community around SketchUp. We haven't got that many active projects yet, still early days. But some reflections on what we've experience so far:
Have clear getting started guides (Even for some of the basics, if only a few links to better tutorials. Open source attracts people with all ranges of experience. It might some times seem like a hazzle to account for the people with almost not experience, but these people, enthusiastic people, can often be quick learners and a great asset if only they are guided a little extra in the beginning.)
Clear style guides. (I see there are services one can hook up to perform analysis of code style that can help reduce some of the back and forth communication on this topic.)
Clear branch guide. (Git-Flow or any other patterns)
Clear guides for level of tests for each contribution.