Lately I installed Gentoo and a couple Ubuntu variants on my PC, the latter because I administer *ubuntus for users. Personally I've always used the oldest GNU/Linux, so I want to chroot in. It works perfectly with Gentoo (able to update). When I do so with Ubuntu variants, it says the following.

I didn't see those in /etc/group, so what does this mean? Other than this, the chroot seems to work well, including updates, apt, etc. Do the error messages mean anything else needs to be configured for things to work right? Of course, the OS I chroot from doesn't have systemd.
        Next I'll be trying same with Fedora and openSUSE... anything I need to know about the case for these?

I think it just means that the group lists in your different distros don't match exactly. Copy all the /etc/group files into one place and compare them. You can always add groups to the distro that doesn't have them.

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Yes, it was that there are more groups in Slackware.

Also, when I chroot into KDE Neon or Mint and startx, it says permisison to open the virtual console is denied; is there any fix for this? (I won't need it for Gentoo but maybe was going to try Fedora and SUSE).

Of particular concern, however, is that "the correspondence between group-names and group-IDs" match exactly wherever the names are the same.

If, for example, the group named "foo" is #20 in one file and #21 in any other, then you suddenly have a terrific headache on your hands. Because Linux knows nothing of the (conflicting ...) names – only numbers. Whereas humans (and their programs ...?) usually deal only with names.

Exactly the same problem can arise with regard to user IDs.

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The fastest way to check for this problem is to touch (to create ...) an empty file, then cat each of the files into it, then sort that file, then use uniq to eliminate duplicates, then sort it again, then hope against hope that you don't find the same name twice. (Isn't the command-line fun?)

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