http://tortoisesvn.net/about
Using TortoiseSVN would mean everyone would be working on the same XSI project folder/workgroup directory, everything could be neatly organised, backed up/versioned and everyone could remain current with a couple of mouse clicks in Windows Explorer. (It's possible there is a Linux equivalent that might work with this but I'm not sure. Is anyone going to be using Linux for this?)
Video about how it works: http://www.vimeo.com/8370887 (Not watched it all but it looks like it shows how it works at about 1m20s).
And we could also use a bug tracker to create a list of tasks, assign people these tasks and keep an eye on what everyone is doing (or not doing
http://www.mantisbt.org/demo/my_view_page.php
It works pretty well for the Black Mesa HL2 Mod. But it will probably cost money (renting server space) to do it this way.