ATLAS was not originally developed in with version control. ATLAS was developed using a programming tool called extract, which means ATLAS is actually maintained in something called basefiles. If you think of regular development being access by value, and version control using a level of indirection, version control on basefiles gives you two such levels of indirection. So, if you want to be able to submit directly to the repository, you will need to learn at least the basics of extract. Details on extract can be found at:
http://www.cs.utsa.edu/~whaley/extract/Extract.html
Note that if you just want read-only access, the install process will use extract, but you will not need to learn anything about it.