Method 1: User A shares with User B
userA > tmux -S /tmp/sharedTmuxSocket
userA > chmod 777 /tmp/sharedTmuxSocket
userB > tmux -S /tmp/sharedTmuxSocket attach
Method 2: User A joins User B
userB > tmux
userA > sudo su userB
userA > tmux attach
Method 3: User A forces into User B
userB > tmux
userA > ls -al /tmp/ (looks for socket owned by userB)
userA > sudo chmod 777 /tmp/userBsocket
userA > tmux -S /tmp/userBsocket attach