How do you know this?
I don't think they necessarily interview everyone they admit. I sense that it's more informal than that, perhaps on a case-by-case basis.
I received the same email today and will likely go. I don't think the email explained logistics for purchasing tickets. Hopefully they'll explain that after you RSVP.
I also applied to UCSB geography. The professor I applied to work with said that the first acceptances will be sent out in late Jauary or early February. They have an invite-only recruitment weekend at the end of February.