How to setup web users in Plesk
1) This is possible using a feature known as Web Users. Go to Websites & Domains.
2) Click Web Users.
3) Click Add New Web User.
4) Enter a username and password for connecting to the server over FTP.
5) Choose which scripting languages to support for this user.
6) Click OK.
Now you can tell your user the FTP account credentials, so that he or she can publish his or her web page.