Login at SuperAccount Control Panel with your main account's username and password.
Once logged in, go to the Website link on the left hand side.
Click on the FTP link.
Click on Manage Users.
Under Add a New FTP User, click on the Add Button.
First select the directory you want to give access to from the drop down list.
Then select what access you want to give them.
Then type in the username and password that you want for this directory.
Then click on the button "Add FTP user".
You have now add a new FTP user to access your website.