Recipe from forums: http://forums.dropbox.com/topic.php?id=3329&replies=5#post-26819
the following 3 files make up the dropbox extension for nautilus
I think they default to standard permissions of 644, with root as the owner and the group set to the root group. If you were to create a new group, like Dropbox, use chown to set the dropbox group on the file, then use chmod to change the permissions to 640. Now only root and members of the dropbox group can see the extension, and thus use dropbox. Then just add people you want to be able to run Dropbox to the Dropbox group.