Aha!

Dawned on my numbed grey matter to seek the reply under Ubuntu 12.04, not just Mint 13. This kind chap provided the answer:

http://maketecheasier.com/add-remove-user-to-groups-in-ubuntu/2012/07/30

The short story:

For some bizarre reason, Ubuntu 12.04 no longer has a Users and Groups application. One either has to get clever with the command-line:

sudo usermod -a -G group username

using your desired group and username as appropriate (dire warning that getting this wrong by forgetting to specify the relevant Group will actually remove the user, which would be fun when you only have one on the system),

or install a cowardly GUI - controlled application (my preferred method):

sudo apt-get install gnome-system-tools

Why on earth omit such a useful tool - which was included in earlier versions of Ubuntu, and therefore Mint?

Windows 8 still refuses to install on VBox, but that's perhaps no great loss. Might bung it on an actual hard drive instead. I tried the last free download - Consumer Preview, I think - and thought it might be worth trying the pre-release "taster."

Be interesting to see how they're doing with TIFKAM (The Interface Formerly Known as Metro...)