A git repository is configured to be shared between users from the same group with:
# grep group /srv/repos/git/l2mesh.git/config
shared = group
The repository directory is set with the group sticky bit so that files created by a user have the same group and permissions as the directory:
root@redmine ~# ls -ld /srv/repos/git/l2mesh.git/config -rw-rw-r-- 1 root git 82 Nov 23 13:09 /srv/repos/git/l2mesh.git/config
The repository can then be cloned with
git clone ssh://redmine.the.re/srv/repos/git/l2mesh.git