I would like to know about this too! I also need to approve specific people to join, but I don't want to have to have a paywall - that seems like too much. What I may end up doing is have a "registration" page that is unlinked on the site, and have the "Digital product" be free. But yes, only trouble is that if they share this login page, someone could have immediate access to everything.
I would LOVE an approval workflow for member sign-ups on specific member sites.