Small Joomla! user management question

I'm working in Joomla! 1.5 and I have multiple groups of users set up.
Now if I set one of those groups to 'author', they can edit every article that is viewable by them.

What I want to do is give this group permission to edit ONLY ONE specific article.

Is this possible? If so, please help me (need to have it sorted out by tomorrow )

Thanks!