Hi Skip!
First, a bit of admin. Mailman 2 is still in active use, so we've split out a separate list for Mailman 3: mailman-users@mailman3.org. Subscribe at https://lists.mailman3.org/mailman3/lists/mailman-users.mailman3.org/ You can use several common social media to authenticate yourself. You'll have to do the usual one-time key dance to verify your email address (sorry!) You will get good feedback from the users there (better than the OP list).
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Skip Montanaro writes:
So, I got to thinking... Are there any free web user (not admin) interfaces which integrate with MM3?
Yes. The HyperKitty archiver, which is part of the Mailman 3 suite[1], provides an interface for submitting posts as well as reading them, and has a few social media features (liking posts).
Something which when combined with MM3 could conceivably be an alternative to Google Groups, (the now defunct) Yahoo! Groups, or Groups.o?
That's for your users to say. ;-) If you visit https://lists.mailman3.org/archives/list/mailman-users@mailman3.org/ you can try it for yourself. To post, you'll need to sign up (you can do that with a Google account, and I think both GitHub and GitLab are accepted as well.
I poked around the archives a bit and browsed list.org for a few minutes, but turned up nothing. Or rather, all I found related to web interfaces was the admin interface.
HyperKitty issues mostly have to do with importing archives. The webservice being effectively synchronous and centralized doesn't have a lot of problems as compared to email, and many feature suggestions go directly to the tracker for HyperKitty. I'm not surprised you don't find much about HyperKitty as Web UI in the list archives.