What you want is called Source Control. It's a way for all of you to work in the same project, different (or even the same!) files.
The one designed by Microsoft is called Team Foundation Server, and they are typically expensive for professional outlets, but Microsoft has something which should provide the level of service you need (way more features than you need, probably). The best part about it though is that it integrates seamlessly into Visual Studio. http://tfs.visualstudio.com/
Alternatively there are other free options, such as github, but they often require your software to obey certain rules (be open source, for instance). Check them out, too. https://github.com/