BBM Beta link was revealed days back, but one could’t install it without signing up to the official Beta program. But today a signup link for the Beta program has been posted by Nawzil. Now you can sign up on BlackBerry Beta Zone site for being able to install the BBM Beta on your Windows Phone.
While you can sign up with any email Id, you will ultimately need to furnish you Windows Phone account Id for joining the Beta. Also, it seems once they accept all Beta testers then only they will send the links, so a bit of patience is needed. Here is the sign up link.