Assuming a group called "sharegroup", a socket called "shareds", and a sessionname called "shared",

To start a new session to share, do:

$ tmux -S /tmp/shareds new -s shared
$ chgrp sharegroup /tmp/shareds
$ chmod g+rwx /tmp/shareds

To attach to the session, if it is already running, do:

$ tmux -S /tmp/shareds attach -t shared

please note that only members of the group "sharedgroup" may access the tmux session. You may have to add users to the group.