This is something we're actively working on! :) "Share by identity" (i.e. to a Sandstorm user, or a Github user, or a Google account, or an email address, rather than by secret link) is high on the todo list. "Folders" are something that will be naturally enabled by the Powerbox, which allows apps to connect to each other -- a "folder" could actually be a very simple app whose purpose is to hold a set of capabilities to other grains, implicitly giving anyone who access the folder access to the contents.
(FWIW, back when I was at Google I spent a couple years working on Google Docs sharing, so I have a pretty good idea of what is needed here. Just have to write code.)