Thanks for your reply. On my system, it runs just as it does on yours. If i am logged into X as a regular user and run the Users and Groups applet, it asks for the root password. After the root password is given, the applet comes up with the regular user shown.  The root account is not shown, which is okay with me in this situation. If I log into X as root and I try to run the applet, the applet window comes up but it is grayed out and a spinning circle is shown when the cursor is over the applet. I can't close it with the close button. I use the MATE System Monitor to end the mate-users-admin process and kill the applet.

Originally in GNOME and MATE, I could log in as root and run the applet.  When I could no longer do it in Ubuntu and complained about it, I was told in the Ubuntu Forums that it was part of their security model and that it had to do with Policy Kit.  When I showed them a situation where it was useful to run the applet as root in X, the post was deleted/hidden so that the forum users couldn't view it. To the credit of the Mint Forum administrators, they have not deleted/hidden a similar post.

https://forums.linuxmint.com/viewtopic. … 6#p1405476