This is what Cindy "AbigaleSS" Bowens (MAG community manager) has to say about beta emails:
Hi, All-
By now you know that there were Beta registration emails sent out today. I've been getting swamped with questions about it.
As to how the recipients were chosen, I have been told by the Beta team that they were chosen based on their standard "selection criteria." I'm not exactly sure what the details of that criteria are but the Beta team has lots of experience choosing these groups and getting a good cross section of testers.
If you didn't get an email today, don't despair yet. That's all the emails going out for now but this is only the first stage of Beta and there will be more opportunities to participate in the MAG Public Beta.
Thank you so much for your enthusiasm and your willingness to help us test MAG!