I\'ve just installed Git using http://windows.github.com/
It seems to be installed correctly but I\'m unable to create new users and access control setup to access repos
GitHub for Windows is a GitHub client for Windows, it's not a Git installer for Windows.
You installed a client that helps you to manage, commit, push and pull from and to your GitHub hosted Git repositories.
If you want to install Git on your machine go to git-scm. If you want to install a git repository hosting, things are getting a little bit more complex, given that you want to do that on Windows. I don't think there is any existing project designed to run on Windows.
If you can use different OS, then Gitolite is one of the most famous projects.
If you prefer hosted solutions, there are several alternatives such as GitHub, BitBucket, CodeBase and many more.