Zenimax has posted a short note on the official Elder Scrolls Online site. According to the post, those who have been invited are all being asked to attend to, in essence, help test the server structure and stability.
We’re getting closer to 4.4.14, when The Elder Scrolls Online officially launches for PC and Mac, and we’re working hard to ensure that the first steps of your adventure in Tamriel are as smooth as possible. This weekend’s test is a key part of those preparations—we’ve invited a massive number of players worldwide to this session to help us get ready and test all of our systems at high volume.
If you were invited, you’ll be actively helping us make launch better with your participation. We need you to get in there, be persistent even when you encounter issues, and push all of our systems hard. We’ll be doing a number of our own tests during the session, like bringing the server down and back up, to gauge how our systems respond to these situations. With your help, we can uncover problems and solve them before launch.
Read more on the Elder Scrolls Online site.