Joining, to me, implies an active decision to take part. Facebook removes that free will and automatically adds people to Groups without their permission at the suggestion of another person. I could start a group right now called "Child Molesters of the Taliban" and put all of you in it without your consent. That doesn't mean you "joined." Hell, a good many of those former wotmaniacs might not even have logged in yet to discover they're in the group.

Thankfully, clicking "Leave Group" prevents people from being able to put you back in it, but Lord, what a pain in the ass Facebook policy.