Pretty sure it's only enabled in the BNet client if you've been accepted/invited. You might not have signed up if you 'won' an invite through your friendslist - there was a promotion blizzard was doing for testers where if they were level 20 before a certain date in November they were randomly entered into a drawing where they could win invites for their entire friendlist.
I got invited like two weeks ago. It's not bad, but it's not amazing either. I feel like there's still something missing - I like gold/items being removed and the effect it has on balance but at the same time it makes it feel like once you find a build you enjoy for a given character, it turns out the same way almost every time since you can't really make changes without noticeably altering your character - there's not really a way to enhance what it can do without also changing it, it's kind of a zero-sum game. Which gets a little stale imo.