Do organisations like that generally use platforms like Lemmy? Do they have a presence on Reddit for example?

I’d have thought that platforms like Mastodon (or Twitter) were more suited to things like making announcements.

I suppose forum-like platforms (like Lemmy) could be used for discussion and coordination of initiatives, I just haven’t seen it used like that myself. I would think they’d mostly want at least a semi-private space for that (e.g. an old-school dedicated forum, or a private Facebook group), not just on the open internet like Lemmy.