It seems Orochimaru left before he appeared in the Chuunin exam (He was acting on his own accord - otherwise he'd be much more interested in Naruto and the Kyuubi). So I'd say that the nine they mentioned later on in the series was an updated number, after Orochimaru left.

And I'd definitely have to say that if Orochimaru left, if Kabuto ever -was- part of it (I doubt he was), then he probably followed and left with Orochimaru.

I haven't really heard many theories. I've heard al ot of people saying they think that all the members have probably been seen already, just not revealed, though.