That's not his term for it, but he attributes to this group of people the rebelliousness and weariness with Rome that ultimately led to the Great War against Rome in 66 to 70, eventuating in the destruction of the Temple. It's hard to tell exactly how close Josephus' descriptions are to what these groups actually believed and thought. The Sadducees are usually associated with aristocratic Priests, therefore they're in Jerusalem. They seem to not have thought that there was resurrection of the dead, which by this period is almost a normative belief in Judaism. And, since they were Priests, much of their religious interests focused on the smooth operation of the Temple, as is right, because that was their responsibility.
Priests are family groups in Judaism.
If you have a friend named Cohen, that means he's a priest. So one is born a priest. One can't choose to become a priest, unlike most other religious groupings in antiquity. But, if somebody is born a priest, he could decide to interpret the Bible according to a Pharisaic tradition, and that's what happened. These are not absolute boundaries.