Usually one of the following ways: 1) know someone who is already on the inside, who has earned enough reputation to get some invites, or 2) Idle in some IRC

Usually one of the following ways: 1) know someone who is already on the inside, who has earned enough reputation to get some invites, or 2) Idle in some IRC channel for anywhere from a few days to a few months, and then pass some quiz/interview to make sure you know the basics of torrent etiquette and such