@MadeInGermany, thanks for that, I will try it later…
And @hicksd8 ‘guest’ in my Mint 22.3 does not appear as a temporary /home/guest directory but in the /tmp directory as /tmp/...

Well, Linus has spoken. Submitting AI Generated code is "OK" but a human must stand behind every submitted line and take responsiblity for the code:

No, guest is not a sudoer (i.e. member of "sudo" group), may not run sudo.

su is different.
In my Mint 21.3 I can make the "sudo" restriction for su in /etc/pam.d/su with

Then the error message changes to

Still first it asks for the PW, so it's anothe...

Ok, then, is guest a sudoser?

Please post content of sudoers file.

Misconfiguration can be the cause.

Sure.
The point is, the su to barry should not be granted at all.
My box asks for the target user's PW, then gives the "cannot set groups" error. Still a bit strange, but there is a second barrier.
A first barrier would make more sense, e.g. configure the PAM su to require the "wheel" group.