Call the grad admissions coordinator to get this straightened out. It may be a system glitch or, if not, the program may be forgiving and let you submit some minor materials late.
If you can't contact them for whatever reason, remember that the procedures for interviews and admissions dates vary widely between schools. Some schools send out interview invites in bulk while faculty send out individual emails for others. If the website doesn't have any information on which is the case, you can only hang tight. However, I would not use other students as a guide unless they are applying to the same schools as you.
Also, and it varies by schools and discipline as well, I have heard that some places actually only invite students they are unsure of. I can't see that happening much with in-person interviews, which can be time-consuming and expensive, but you can't rule it out either until you have more information.