@codinghorror has made it pretty clear he’s not interested in sending digests to people who log in, and it never worked that way.
So I suspect this is plugin territory.
The workaround is for admins to send out weekly digests to everyone manually - the digest preview is pretty amazing for making this quick and easy to do. Adding a group/username filter to the preview to see what real world users actually would see if they got the digest would make this incredible feature even more useful!