They will get a notification that someone shared them on a draft, and the post will show up below their own drafts on their personal profile page (only visible to them, obviously). However, until we ship some of the inline-commenting features we've been working on, there isn't actually a way for them to leave feedback on the post, which is unfortunate (we used to allow users to leave comments on drafts, but there were various problems with that, among them it being unclear whether those comments should persist to the published version of the post or not). 

So that means that while they will get notifications about being shared, you should probably PM them if you want them to leave feedback. We do have an editor in beta that will allow users who are shared on a draft to leave Google Docs style inline comments, which I expect to go up within the next few months.