You already could set up private groups; this is just a way to get families to do it. Facebook may be trying to capitalize on the success of genealogy applications such as FamilyLinks Were Related and FamilyBuilders FamilyTree.
The landing page is here (youll have to log in to Facebook if youre not already). It lets you name your family group and invite relatives already on Facebook and those not yet on Facebook. Then you can share photos and information just with this group.