Auto-tagging a Chatter group in feed comment

We want to do exactly what the PowerofUs hub does when a user posts a question and automatically add a threaded comment tagging a chatter group.

So for example, when I post this question here, PowerOfUs will then add a comment tagging the "System Administrators" group. How can we replicate this functionality in our instance?

I've tried with the Chatter Post apex action but that only supports posts, not threaded comments. I also tried by creating a record of type "Feed Comment" but when adding the @ tag, it just gets posted as plain text rather than actually tagging the chatter group.