Try this to make an existing repository in repodir
work for users in group foo
:
chgrp -R foo repodir # set the group
chmod -R g+rw repodir # allow the group to read/write
chmod g+s `find repodir -type d` # new files get group id of directory
git init --bare --shared=all repodir # sets some important variables in repodir/config ("core.sharedRepository=2" and "receive.denyNonFastforwards=true")