I'm looking for something similar to the
screen -x of gnu screen for Linux
You can attach to tmux sessions owned by different users by giving the correct permissions and doing the following:
For example, to set the permissions for all users:
It will work for you by simply attaching the same session, with no special switch.
One of the cool advantages of tmux for shared sessions is that it automatically adjusts the pane sizes to the smallest actual terminal size of all of the currently connected clients.