Thanks guys. I was indeed limited by Raspbian Stretch, which didn't allow me to update Calibre. After upgrading to Buster, I was able to update Calibre to v.3.39, and gain access to the user management functions. Yeah!
(as a small enhancement to the calibre-server, I'd recommend that users that aren't able to upload books or edit the library don't see the "upload" or "edit" buttons on the interface, as they still appear on my end. But perhaps that was corrected in later versions? I tried checking the changelog but drowned a bit! Which shows what an actively developed this software is - bravo again
)