It is not always necessary to register a new domain name when the one you already own will work perfectly fine. Rather than registering a new domain name, you can always create sub domain using the domain you already own.
This video demonstrates how to get the more mileage out of a domain name by simply creating subdomains within your cPanel. The ability to create unlimited subdomain is another great feature that comes a standard offering within our hosting plans. Check it out for your self at http://hostgator.com
Additional tip: For those of looking to install WordPress multiuser, just follow the instructions in this video to create your wildcard sub domain. Rather than typing in an actual name, use the asterisk symbol (*) and let your WordPress MU installation will work it’s magic!