We have a plugin for this (though it doesn’t look like we have it documented here on Meta anywhere 
): GitHub - discourse/discourse-group-tracker: Group Tracker plugin for Discourse
This allows you to add an icon to the topic list to mark topics where someone from a specific group has posted, and also covers this:
Also, within topics… as long as the group is set as someone’s primary group, we add a class to each post (using Meta as an example, our team members have group-team added to their posts:
