Sharing a single Dropbox between multiple accounts on one Mac - possible?
You can do this using ACLs, which will avoid permissions problems when new files are created:

sudo chmod -R +a "$USER allow read,write,append,delete,list,search,add_subdirectory,delete_child,file_inherit,directory_inherit" "$DIR"


where $USER is the username of the other user, and $DIR is whatever directory (Dropbox or otherwise) you want the other user to have full permissions in.

We use this to share a single Dropbox folder between four users (2 adults with full access, 2 kids with access to their subdirectory only), and it has been working well for us.  The only limitation is that the user running Dropbox has to be logged in and running Dropbox.  The other users don't have the Dropbox icon and status info in the menu bar.