It would be a sort of spin-off of an existing Facebook group. Facebook is not really good because, well, it's facebook. But also too much extraeneous clutter, not good for uploading images, means people have to be linked to personal profiles, and I don't like the forked discussion threads and un-searchability of everything.
i agree that work and farcebook shouldn't mix unless people really want to. place i worked during the covid years got a farcebook group up and running, initially to keep in touch with people who were on furlough, and then it replaced some things that would have happened in staff meetings.
it got a bit unpleasant and there were fallings-out and HR imposing a code of conduct, and people being banned from it.
one or two colleagues really didn't like having their off-duty time interrupted with being tagged about things that people thought were their fault.
and there's people who might not want to mix every aspect of their social / personal life with work. (i certainly wouldn't even if was on farcebook - i would not want my tweeter or urban identities to be linked to my work identity)
MD wanted me to go on there and since i don't even have a farcebook account, i said no, it would be like putting my home phone number on work e-mails. if they wanted to set up an exclusive forum for staff, i'd go on there (one place i worked did the 'yammer' thing but no idea how much that costs. this was a pretty big organisation.)
I also use a few io.groups, basically where old yahoo groups migrated. That potentially could work ok, except it's not really optimised for posting images.
groups.io is ok-ish, i'm on there for one personal thing and (with work not personal e-mail address) something work related (based on a trade organisation rather than just one employer.)
it's not free except the very basic version and any images / documents get added as an attachment.
you have the option to get individual e-mails for every post, daily (or every X number of posts) 'digest' e-mails, or just go on and use it like a forum. I think either of the first two would annoy the heck out of me, bu tit does mean you can reply to an e-mail and that ends up on the group.
the 'digest' approach also means that the thread doesn't always follow in real time, and it's not always obvious what people are quoting when they reply to something from a while back.
another snag of doing it as an e-mail exchange is if your work e-mail system adds to the e-mail subject then any reply goes off the main thread unless you edit the subject carefully to stop threads getting fragmented, e.g. there's one thread re:stuff and another re:[external] stuff
I use Basecamp a bit (that's something hosted by relevant government department) which seems similar but with more gimmicks and no idea how much it costs. And someone's out of office reply managed to get through to one thread today...