If you are using Plesk 7.6 or newer, which you should be, you should be able to create FTP subaccounts for domain access under "FTP Management" icon. If you don't see the "Add New FTP Subaccount" icon there, you need to correct the domain limits to allow creation of FTP subaccounts.
I hope this helps.
I'm using Plesk 7.6.1. I do not not have a link to create FTP subaccounts on my FTP Management page. From the domain mydomain.com page I clicked on the Limits icon in the Domain section. This was a display only page with no options to change anything and NO rerferences to FTP anything.
My desire to is give specific people FTP access to specific folders on my site. Can I do this?