How to add a Domain?

If they have their own domain then issue them their own cpanel. In whm, account functions > Create a new account.

 

If you have access only to cPanel, most probably you must use the Addon domains option.

 

Ask your hosting provide if you have addon domain feature in cpanel
Addon domains allow you to host multiple domain names from one hosting account, utilising the extra space and bandwidth available on your main hosting account. Addon domains point to a subdirectory within your main hosting account. From a web user's perspective, an add-on domain functions just like any other domain name, and the user will not know the site is hosted from your main account. The browser's address bar will show 'http://www.add-on-domain.com' (no reference to your main domain name). Addon domains are created through your control panel (check to see if your hosting package includes them).
Steps
1 To access the Addon Domain Maintenance menu, click on the icon above the words Addon Domains on the main screen of your cPanel interface.

2 Enter the name of the addon domain in the blank field next to New Domain Name:

3 Enter the name of the directory which conatins the subdomain that the addon domain will point to in the blank field next to Username/directory/subdomain Name: (This directory will be the location for the addon domain's files, just as public_html is the location for your domain's files.)

4 Type in the password for the new addon domain, and click on Add Domain!



NOTE: You can upload files directly to this addon domain using the username and password for the addon domain to log in through ftp.

 

I was once a SiteGround customer and just so you know you will probably have to be pay extra to have addon domains.

As well, just submit a support ticket. They allow you to have 7 free ones a year so you might as well use it.