That is not possible. Just setup each person with their own FTP user, and if you ever need to manage the files of a particular person, just login with their FTP username and password. Simple as that. You can always retrieve their password as long as you created the user on a plan managed by your WebId.
Anonymous FTP is where visitors don't need to have an FTP user account of their own - they login to FTP with the username 'anonymous' and an email address as the password. It's used for allowing others to download files using the FTP protocol instead of HTTP.
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