Adding domains to your domain group is very easy. Go to "Domains", select your domain group and click on the "Add Domains" tab. 

Enter your domains and click "Next ". Select the IP address, hosting, choose if you wish SSL to be installed immediately, the software to install, and the protocol to use for your software.. 

You can install SSL, software and set redirects even after adding the domain as well.

Note that you can add multiple domains at once using this feature. If you're intending to add more than one domain at once, add one domain per line.