This assumes you have your machine set up and have screen installed;
Find out if Screen is installed 'setuid' or not, run; 'ls -l $(which screen)' and look for an 's' in the permissions field, like '-rwxr-sr-x'.
If you don't see that 's', run 'sudo chmod u+s /usr/bin/screen' and 'sudo chmod 755 /var/run/screen'.
As the user that want's to share a screen (amy in this case), run 'screen -d -m -S shared ; screen -S shared -X multiuser on ; screen -S shared -X acladd bob'
Give the user bob instruction to run 'screen -x amy/shared'
At this point, both Amy and Bob should see the same screen, either user can type - if Amy runs something like 'sudo echo “Hello”', she will be able to type her password, sudo will hide it from both users (it may echo the asterisk character).
Note that if two people are logging into the same account1), you don't need the 'acladd' bit.