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http://www.uvmedia.com/how-git-open-atrium

How to Git Open Atrium | UV Media

UPDATE: "Open Atrium has moved from its single monolitic repository to an install profile and a drush make based build system. This is a change that primarily effects developers. The complete, ready to install, download will continue to be offered at http://openatrium.com". Details here.
Introduction This article introduces version control and Git without assuming you have any prior knowledge or programming experience. Because of its introductory nature, certain details are simplified or omitted and the use of the Git Graphical User Interface ( Git GUI ) is emphasized.

Git: Your New Best Friend [Server Side Essentials]

http://www.sitepoint.com/version-control-git/