sharing between different account involves using a "socket" -- a special file that can be configured to be sharable.
tmux -S /tmp/shared new -s shared chgrp users /tmp/shared
Where shared is the name of the session, and users the name of a group that everybody is already a part of. See groups.
For the **others**, you would connect with:
tmux -S /tmp/shared attach -t shared -r
For a read-only session, or:
tmux -S /tmp/shared attach -t shared
For a full session.