You already are, if you have a Battle.net account and have completed a Beta profile. Blizzard maintains a support page with detailed instructions on accomplishing this.
In order to be eligible to participate in future beta tests, you must create a beta profile. A beta profile is a snapshot of your current system specifications. It includes such information as how much RAM you have, available hard drive space, your graphics card and driver, and other information about your system. We gather this information to ensure that we are able to test our games on a wide range and distribution of systems.
You can see more information about this in Blizzard's Beta Opt-in FAQ.
