To make screen run as suid root, as the root user do:
chmod +s /usr/bin/screen

Then:
USER1:
Start a screen session by typing "screen" at the terminal.
Press ctrl+a and then the colon (":") key to enter commands.
when you do that, you'll see a highlighted colon prompt at the bottom of the
screen.
Then turn on multiuser by typing: "multiuser on"
and then grant permission by hitting ctrl+a and typing "acladd USER2".

USER2:
Can now connect to the USER1 screen by typing "screen -x USER1/