One user, multiple accounts

Thats correct each account must have a distinct username as when the account is created a folder with that username is created under /home and all their files are stored their the user name is also used as the prefix for MySQL datbases names and usernames.

If they want to only login in one place then they will have to be setup as a reseller with all the other accounts under theirs, as resellers they can log in to their cPanel account on the server and they will have a drop box so they can jump from one account to another to access the features, they will also have access to WHM but you can set them up with limited functions so that they can only access certain features if you wish to restrict them.