To create multiple FTP users for a site, you create what are called "web users" from the Plesk control panel. Each one will have its own unique username and password. Each will also have its own separate web space on the domain.
Keep in mind that only one main FTP user can be set up to manage the main site content for each site. Use the online help within the control panel for additional configuration information.
If you wish to give users access to a specified directory in your domain (so they can make changes to specific files and folders without having root access), you can create a subdomain and give the new users access to that subdomain via the main FTP account for that subdomain. Then link to the subdomain instead of using a directory off the main site. This way a user can FTP to that subdomain and without making unwanted changes to your main site. This method is also more search engine-friendly than using subdirectories.
Otherwise, directly supervise the users when they are using FTP on your main site to be sure they don't do anything undesirable. It is also a good idea to make a backup of your site before allowing new users to work directly on your main site.