The IRC channel #xubuntu-devel is where most of the daily communication and cooperation about Xubuntu development happens. It's also the place to get to know people and get yourself known.
The development for the current Xubuntu release is tracked in the Xubuntu development tracker. In addition to showing the current status for blueprints and their individual work items, the tracker displays some general progress. Finally, you can access the team calendar, Xubuntu wiki, development mailing list and join the development IRC channel #xubuntu-devel from it.