They have to be reported to have either left China or be in an embassy or other way extraterritorial area, or actively be in flight.

If they attempt to set up an alternative government or similar within China, that counts as long as the former core is pursuing them.

There are 24 active members and more, olde, former members.

https://en.wikipedia.org/wiki/Politburo_of_the_Chinese_Communist_Party?wprov=sfla1