These steps are from this page: http://www.spinellis.gr/blog/20130619/

• Create a group for git users: groupadd gitgroup
• Add each user with “useradd -m -G gitgroup <username>”
• Create the repo in a place which has at least group/user level rx permissions (don't need w):

    # cd <destination directory>
    # git init --bare <repo_name>.git
    # chmod g+rx <repo_name>.git
    # chown -R :gitgroup <repo_name>.git
    # cd <repo_name>.git
    # git config core.sharedRepository group
    # find . -type d -print0 | xargs -0 chmod 2770
    # find . -type f -print0 | xargs -0 chmod g=u

• Clients can now login with SSH authentication and clone the repo