Each domain has it's own FTP Username and password associated with the domain. If you want the user of a specific domain to be able to access the PLESK control panel as well, and only see their domain then you need to set a domain user password for that domain from within the PLESK control panel. Here is the steps will help you.
1. Login with your PLESK Control Panel username. The one that can see all of the domains.
2. Click the domain you wish to grant access.
3. Under the Domain Section, Click the Domain User Icon.