I think, and this is just a thought, that perhaps these people are Uchiha. Since I really doubt that Kakashi was being refered to when Itachi said that "then three people would be capable of using this doujutsu". I also doubt that he was refering to three people in history, because that means that there was only one other user of the Mangekyou, and Itachi's father sid that there have been very few. Few means more than one. I think that this may be a group of Uchiha that left the clan for the same reason that Itachi did. To escape the constraints of the clan and the village to achieve real power. Maybe..?

But on another note, this Sai is stronger than anyone in this generation, meaning Naruto, Neji, and Lee. So we can assume that she's at least at the Jounin level. Curious much.