It depends entirely on the school. Usually if people heard from someone other than directly their POI, it means they all went out at once, though that's not ALWAYS true. I would say your odds are significantly reduced if it's been a week or 2 since people got invited AND the invites were sent from a central person, like a DCT or graduate secretary (not individual POI).
However, if people have heard directly from their POI, all bets are off unless the POI mentions something specific about how other people are getting invites (e.g., all this week, scattered, etc.). Some schools just let the POIs do it as they decide. Most schools do generally give you at LEAST 2 weeks notice on the interview, if not an entire month, so if the interview is less than 2 weeks away, the odds are low.
That's just my opinion on the whole thing.