Note that beta testing is not user testing. I would read up on the background to usability testing. Briefly, there are two types of usability tests - formative and summative - summative research is more when you are developing the product and want to iterate rapidly based on feedback from users. Summative testing is when you want to create a baseline of data for comparison with future releases.

Your beta testers should be using the product over a longer period of time than a usability test, which is typically a one-off session.