I think groups are a pretty half-baked feature, and I don't expect people to use them much. A "group" (previously called a "forum", and most of the documentation hasn't been updated to reflect this change) is basically an account that auto-reshares things it's tagged in. You can give it a few different settings for how it responds to follow requests, and another setting for the visibility of its reshares. To administer a group, you have to create a second account for it (which friendica does make relatively easy but not trivial), and then switch to being logged-in "as" the group.

So, yeah I guess it might be useful for coordinating things somehow, and the setting with private reshares is maybe promising (though also marked "experimental"). But it seems much less natural to me than the corresponding concept on Facebook.