Technically this should go in the Server forum, but I'll answer it anyway.
In Server Admin, click FTP, then Settings, then Advanced. Change "Authenticated users see:" to something that includes Share Points--your choice. I have it set to "FTP Root and Share Points." Then, in Workgroup Manager, add the directory in question as a share point. Of course, check the permissions and make sure that you aren't letting people in that shouldn't be allowed. Otherwise, your web server might get 0wn3d.