Calling polyamory a society that anyone has "joined" is a rather strange way of looking at it. There actually is an organization called The Polyamory Society
, but that's an advocacy group and no one needs to join it to live polyamorously. Read as much as you can. Look at the sticky notes at the tops of our forums here, where you will find suggestions for books, other websites, and relevant discussion threads here, if you want to learn more.