The host user runs:

screen -R paircode -e^tt

-e changes the escape character. paircode is the name of the session.

Then type:

control-t : multiuser on
control-t : acladd guestuser

control-t : takes you to "command line mode". guestuser is the username of the guest user.

The guest user runs:

screen -x -r hostuser/paircode