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