"It's what I call a 'conference call'." said the elderly relative.
I didn't have the heart to mention that that is apart from the facts that: there's no audio conferencing going on; there's no audio; it isn't done via the PSTN; it's desynchronized; there's a reviewable history; and one doesn't even necessarily need to use a 'phone (as there's a desktop application that can clone a sign-in from a 'phone).
So: unlike a conference call in pretty much every major respect.
"Private mailing list" is just as (not very) close to the mark. (-:
Or talk(1). Although then I'd have to explain what BSD was. But not what a terminal is (although that might have had to include an update about how punched cards have been supplanted by these VDU things).
https://man.freebsd.org/cgi/man.cgi?query=talk&sektion=1
IRC would have involved a lengthy explanation, too.