Version Control Systems

You can use a Version Control System (VCS) for your collaborative work by storing there all necessary sources of the map.

The general process and nuances listed above will not change in this case, but the process of exchanging changes will be much simplier.

There is no direct "out-of-the-box" integration with a particular VCS, so you can use a VCS you like: Git, Perforce, Subversion, and so on.