Setup Several Password Protected Directories that you and/or clients can View, Download, and Upload Files

1)
Typically this is used where you have one main Directory that has the Virtual Directory - Directory Browsing, so users can see all Directory Folders in the main Directory.  Directory Browsing Instructions seen here:
http://support.colony1.net/KB/a207/how-to-allow-users-to-see-listing-the-files-subdirectories.aspx 

2)
Then you have subfolders that are Password Protected so users have to login to a sub directory before then can see the files/folders.  Password Protection Instructions seen here:
http://support.colony1.net/KB/a206/how-to-password-protect-directories-in-plesk.aspx 

3)
Finally you also allow users to upload files/folders by having individual FTP accounts for them. FTP Account Instructions seen here:
http://support.colony1.net/KB/a205/how-to-add-ftp-accounts-in-plesk.aspx 


Example, so you could have:
A folder called /Public_Company_Files/ which allows all Files/Directory Browsing to be viewable from 1) above.
Then Sub-Directories under it called /Billing/ etc. that are password protected from 2) above.
Then you can have an FTP account called Billing_FTP from 3) above.

So now This will give you a URL, http:// {Your_Domain} /Public_Company_Files/Billing/, which is Password Protected.
Then a Billing Rep can go to that URL and enter the user/pass to get access and they will be able to see all Files/Directories.
Also the Billing Rep can upload Files to /Billing/ through the Billing_FTP Account.