I honestly don't know when this came on the American Girl website, but the product SKU suggested it was fairly recently. I couldn't find it on the AG wiki either, so I'm going to treat this like it's a brand new thing!
This is a carrier for your dolls! It fits one doll and goes on your back.
While this isn't a good fit for me: I have way more than one doll and, more importantly, a weak back (even though I got surgery on my back, it isn't the strongest muscle in my body!), I'm sure you guys would enjoy it!