Users in HSPC are Linux users. Linux users are global to the VPS, and not to a specific domain. FTP users are Linux users with the special privilege of being allowed to FTP. HSPC does not designate groups to sites.
The typical way to set up your sites with FTP users that are restricted to a particular domain is to create one FTP user for each site (or set of sites) that the user will manage. You then set that user to be the administrative user for each site he will manage.
What this does is create the following directory structure:
where you replace the user# and site# with the actual user and site names.
All files within a site directory are assigned the user and group of the user directory they are a part of. For instance the directory /homeuser1/site1/ has the user:group permissions of user1:user1.
When you create a FTP user, the user's home directory is set to /home/username. If you choose to you can set up a default chroot for each of your users for proftpd. To do this you will need to edit your /etc/proftpd.conf configuration file. For more information see the following link knowledgebase article at: