Ah, well that may not have been quite as rude as it seemed. The thing is a bit laggy, atm, so it isn't always up to date. (Hitting the "x" at the bottom and then selecting "stay" will force it to refresh and be current, which is fine(ish) unless it already started.) But some people have even found themselves late to the battle as a result of the lag, or they hopped off and the screen didn't reflect that. But part of the reason you'll see that happening is there are ways of playing with people you know (it's a little complicated) and it requires hanging out in a chamber and waiting for the others to find and join you.

But then again, there are enough people who just haven't figured out how to push the "join" button... *shrugs*