how is one put into the inner or outer party in the book 1984

The Outer Party is a huge government bureaucracy. They hold positions of trust but are largely responsible for keeping the totalitarian structure of Big Brother functional. The Outer Party numbers around 18 to 19 percent of the population and the Proles make up the rest. The Inner Party is the ruling class, and it communicates Big Brother's message to the lower classes. We never really find out numbers of men in the Inner Party but they are likely few with O'Brien being among the most important members. WE are never really told how one becomes a part of the Inner Party. I imagine one is born into Outer Party or Prole status.