I don't know that you need an invite anymore
Actually I'm pretty sure you do. It was only a few days ago I'd clicked over there on my not-logged-in browser and it has the big ol' "REQUEST INVITE" button front & center. Maybe they're better about actually sending them out now when people request, but I've no idea.
This sounds like fun but I don't have a pinterest account. I would love an invite.
I'd be happy to send you one, just need your email addy to do so.