You use public distribution lists for that... You can use DLs already in Exchange, you just need to import them into Unity on the distribution lists page in the SA... or you can create new ones of course.
You set the message recipient of a call handler to be a public distribution list and messages left for that handler get sent to everyone in that DL (including non subscribers - voice mails will show up as emails with attached WAV files).
Unity creates a new DL called the "All Subscribers" dl which, by default, all subscribers created on the system are added to. You can use this for the "all users" stuff. There is also a Public Distribution List build in the tools depot to help you create DLs from CSV, off of COS membership, extension range or the like.
