You can use a domain administrator login to give full or partial access to just one domain in the plesk control panel, to another person.
To set up a domain administrator login, simply login to siteadmin, as outlined in this tutorial
Click Domain Administrator
Put a tick in the "Allow domain administrator's access" box, and fill in the password, and select what options you wish the domain administrator to be able to access
The domain administrator then simply logs into siteadmin, using the domain name (without the www.) and the password as set just above.