In Book II of Plato’s Republic, Glaucon tells of a shepherd who finds a ring that makes him invisible, and says anyone with it “could safely take what he liked out of the market” (Jowett’s translation). That’s Glaucon’s challenge, not Plato’s view: Socrates spends the rest of the book answering it.
How to play it
Turn the ring on his finger until he vanishes. Then help yourself.
That’s the whole collection examined, every room. Now imagine doing this with your own life, for a year, with two people who come to know it well. That’s membership.