Setup tunnel
    
    ssh -fN -R 2222:localhost:22 serveraddress

Connect
    
    ssh -L 2222:localhost:22 serveraddress

Create a session that can be shared
    
    tmux new -s some_name

Ssh back to localmachien
    
    ssh -p 2222 username@localhost

Invite someone else to log into server

    ssh serveraddress
    
How to connect to the same session

    tmux attach -t some_name