Thanks to both of you! And sorry for putting mess in the support forum. 😕
Unfortunately, I'm not skilled enough to develop my own extension which would ask for a general password before entering the forum for logged-out users.
However, I like clarkwinkelmann's idea, and I have tried to install
johnhearfield/flarum-ext-landing-page extension to display a customized landing page for non-logged users, which would tell my neighbours to sign in with generic login and password (or use their own account).
But I can't make this extension work with my beta14.1 flarum instance, because no three-dot button with Settings appears in my extension page:
I have also tried
kyrne/landing-page forked version, which displays this Settings command, but generates an error at the bottom of my extension page (I have a translated version, but it is something like "there's something wrong, please clear cache and retry", which I have done).
Can you see something obvious which I would have not thought about? Any dependency to satisfy?