1. Log into the Customer Control Panel with your main account's username and password.
2. Once logged in, make sure you're on your VPS subscription at the top right hand corner. On the Home tab, you'll see a link that says "Login via IP Address" under the Parallels Plesk Panel section at the bottom left hand side of the Home page.
3. Once you click on that link, a new window will come up, you might get a certificate warning, just click continue, then your Plesk Panel will come up.
4. Click on "Domains" on the left hand column. Select your domain you'd like to manage.
5. Click on the "FTP Access" icon.
6. Click on the "Add an FTP Account" link at the top.
7. Fill in the username and password and what access you want to give them.
8. Click OK.