A critical part of collaborative work is accountability. And with large projects with much in flux, it is equally important to document what one is doing for one's own sake as well as for others.
To this end, I'd like each of my students to start a "project log" post. This is to be updated at least twice weekly as students account for what they have done both to move forward their individual project, and to contribute to the overall project. Each student will only create this blog post once, but will update that same post, briefly logging their activity and linking to the other blog posts they've completed.
This kind of log is a bit of a hack -- there are other ways of doing this. But it will work, and this will keep each other aware of what one another is doing. (For academic purposes, this will also help grading.) Instructions follow. I have modeled how to do this in my own project log.