Word of caution, if you’re using multisite on a shared hosting account (which isn’t considered a best practice and as your site gains traffic, you’ll want to make the move to a VPS) you may be required to manually add a “parked domain” for each new domain a user wants to use. The domain mapping plugin will match your users domain name to their specific subdomain on your WordPress installation.

No, this won't work in every case. You can park a domain in cpanel ONLY on top your MAIN cpanel domain.

That means if you did not install Wordpress multi user under your main cpanel domain, then don't park the domain you want to add to your network, because it won't work!

Instead, do this: http://support.hostgator.com/articles/cpanel/how-to-park-a-domain-on-an-addon-domain

Every blog post which discuss the domain mapping plugin forget to mention this, and this drive a lot of people nuts.