Annotary. Annotary struck me as being a bit like Diigo meets Pinterest. I liked Annotary but it was missing something. That missing thing was the option to create bookmark sharing groups. This week Annotary added that option.
You can add existing Annotary members to your group or send out email invites to the people that you want to join and use Annotary with you. Annotary provides a signup link you can share that automatically signs visitors up for the group. You can create group collections or move your existing collections into a group. Everyone in the group automatically follows the group's collections and can contribute their own bookmarks and annotations.