Jon_in_NC
Member
The link was shared because the “waiting room” page was loaded on the Disney website. That’s how people found it.
It happens for many preview events. The URL is pretty easy to deduce and when people find that there is a message saying “the waiting room is now open” it gets shared on fan sites, such as this one, and other social media.
Only pass holders were able to register. Sorry you weren’t successful this time.
So, Disney has established a "waiting room" to join before sending out emails notifying that people can join the official queue. In general, it is a horrible, unreasonable system that they use. (see also the Guardians of the Galaxy: Cosmic Rewind previews)