OK, not sure why someone from SE could not post this, but if you have a comcast e-mail address registered for your account then change it to a non comcast e-mail address or you will not get the beta invite. Apparently the beta invites are coming from a company that comcast considers spam ie they probably get paid to send out spam and SE hired them for this job. You'll probably also have to submit a ticket, get a response that they can only help by phone, call them, and ask for the beta invite to be sent to the new e-mail address. Hopefully that will fix the issue.