Maybe reading carefully through the DH WIki article on Users will help. There is a link within that article to a DH Wiki article entitled "How Do I Share Website Access With Another FTP User, which might help further.
There are other approaches you could take, though how appropriate any one of them might be depends a lot one who you are interested in sharing the directory/files with, and for what purpose. Often, FTP is not the best approach for this at all.
If after reviewing those articles, and the related links contained in the articles, if you still need help, post back with more detail as to what you are trying to accomplish and why, and maybe those here can provide additional suggestions that are appropriate for your circumstances.