In my organization the Account object has public read/write on the sharing settings, and my community users can only @mention the system administrators and other people from the same community on chatter. I would like for them to be able to @mention internal users as well. Is it possible?
I would think they can only mention members of the community. I believe you can add members by profile in the community settings. You could add all your internal users to the community this way.
If you think about it, this should be how it works. How else would your internal users be able to respond on chatter?