Bitbucket is an online service used to track progress and changes on collaborative projects. While it was originally designed for software development, it can be used for basically any type of design project. There are also several downloadable programs (including SourceTree) for local storage and tracking of files and progress.
Bitbucket offers free, private projects for small groups (5 users or fewer), but requires a monthly charge for more users.
Bitbucket offers guides via https://bitbucket.org/support