# screen -S screen_name
ctrl-A :addacl add someone_else,and_another_someone_else
ctrl-A :chacl username -w "#"
ctrl-A :multiuser on
They can connect to your screen via
# screen -x owners_user_name/screen_name
Obviously replace owners_user_name with whoever started the first screen, and change someone_else, and_another_someone_else, and screen_name to the useful settings....
More good tricks here, including some fancy tabs for the bottom of your screen