Shared Web Hosting
You will add extra domains through your cPanel feature called "Addon Domains".
Reseller and Dedicated Hosting
You can add extra domains through your WHM feature called "Create a New Account". You can also allow each cPanel user to add extra domains via the "Addon Domains" feature, and the limitation of this feature is decided by you.
Don't forget! You must register a new domain before you can use it with your hosting. All domains added to your account use the same name servers we provided in your welcome email.