One way to do this is to create a group for the "git" users and add all the users who you would like access to this group. You can use the "groupadd" command to create the group, then I just edit /etc/group to modify the comma-separated list of users in that group. I usually name the group based on project, like "newwebsite" rather than something like "git". For example, the /etc/group entry may look like:
Now, create a directory to hold the git repository, say "/var/git/newwebsite": mkdir -p /var/git/newwebsite
Set up the permissions on this directory to include group sticky bit:
chgrp newwebsite /var/git/newwebsite
chmod g+ws /var/git/newwebsite
Now initialize the repository in this directory with:
git init --bare --shared=group
NOTE: Do not replace "group" with your group, it needs to be the literal string "group".
If this errors out, your version of "git" probably doesn't support the "--bare" option. Remove the "--bare" option in that case.
At this point, your users should be able to check out the repository with:
git clone git+ssh://$USER@$HOST/var/git/newwebsite