Sin, as opposed to “Perfect,” means to “miss the mark; to fail.”
Groups are communities of people with similar interests or lifestyles in common. Browse around! Some are private so they require acceptance by the moderator - but those are just to make sure, for instance, that you are a Woman wanting to join the Woman group. This should be obvious by your username if you logged in with Social Media. Don't see a group you would like? Create your own!