Can you self-host, or are you looking for another online service? Facebook Groups is basically a forum when it comes down to it, and any forum software can do what you're asking. I really like Discourse. You can self-host it for free (well, whatever your server costs), they'll host it for free if you're an open-source project, or if you're a legal non-profit you can get 50% off their hosting for $25-50/month.
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How about Friendica
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Matrix. It's very secure and allows you to view several group chats/topics at once, and you can video/audio call through the Jira app extension.
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Does it also offer features to pin important files/messages?
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No but the most conversed topics stay at the top of the feed. Also my bad, it's now called Element in the app store, I forgot about the change.
I would challenge "very secure" but it is more than secure enough for this usecase.
Ehhh unfortunately I don't think there is a good post-style alternative. Probably your best bet is a Discord-style multi-group-chat system like Revolt or possibly Matrix Element.
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