An add-on domain will direct its visitors to a subdomain on your account. You can add your other domains as add-on domains to your hosting account and share your hosting account's resources.
Before you can add a domain as an add-on domain, you will need to change its name servers (DNS) to Doteasy first. The name servers for add-on domains can be found by logging into Member Zone, and clicking on the View DNS Info link under the Domain Settings section. If you're logging into your Master Account, you will need to click on the domain name first before you can see the this option.
Once you've done so, please follow the instructions that are provided in this video to setup your add-on domains: