Screen -x

To start using multiuser mode, use the command multiuser on in either your .screenrc or on screen's command line. Then use acladd or aclchg to tell screen about the users that you want to connect. screen will not allow connections from any user it hasn't been explicitly told about. Once the permissions are set up, the other users run screen -r / to attach. (If you want multiple people (including you) to be attached at the same time, use -x instead of -r. If you have multiple sessions, put the session name after the slash.)

http://aperiodic.net/screen/multiuser