@scott@loves.tech can you share an example of a group actor from Hubzilla? Would be interesting to see how that's handled. Likely it wouldn't work properly because categories in NodeBB don't author posts.
Do your group actors send creates on behalf of regular users? That might work ok.
Lastly, there's no requirement that a NodeBB category be mentioned. It only needs to be addressed. A mention is the easiest way to do that because you addressing is abstracted out of the Mastodon UI.
But for things like PieFed, Lemmy, Mbin, and likely Hubzilla, you're able to change addressing based on where you create the post.