In a nutshell screen is ...
Hmm... single-user is great but for multi-user it requires chmods which can be security vulnerabilities.
http://packages.ubuntu.com/maverick/gems - sounds like a good alternative... NOT, looks abandoned.
Back to screens, this looks good - http://blog.bodhizazen.net/linux/shared-ssh-sessions-update-for-jaunty-ubuntu-904/ - original article http://blog.bodhizazen.net/linux/shared-terminal-sessions-over-ssh/
Resources
http://www.pixelbeat.org/docs/screen/ - looks good
http://www.roboguys.com/index.php?option=com_content&task=view&id=79&Itemid=47 - looks like a nice simple tutorial