In another thread it was indicated that its easier to do complex things with the dataset ACL type set to NFSv4 and not POSIX.I only got this far with ACL:
I was able to save ACL permissions with user/group selected and assigned to "USERA" on the left side (File Information) AND UserObj; GroupObj; Other on the right side (ACL).
Given that I did not set permissions for Other, I can only connect to this share with USERA. That's as expected.
I cannot, however, add additional (read-only) permissions for another user "USERB". With varying error messages that do not necessarily make sense to me I cannot save any changes to my above maintained permissions.