Login to Cpanel and click on FTP Accounts in the Files section
You will notice there is already a couple of users since the creation of your account
To add a new FTP account enter the username
Then enter a password
Re-enter the password here. If you leave / demo directory in the box, then the user of this account will have FTP access to the directory / demo and all records relating there
For the user to have access to the entire website, remove all but the slash "/". If you would like you can limit the disk space allowed by entering the Quota in the field. Once done press create.