Update: Just realized that this could be an old feature but I can only see it in one of the five Facebook Groups I am subscribed to right now.
Facebook is rolling out basic public analytics on their Groups platform. All members of the group would be able to see the names of members who has seen individual posts.
This comes in handy for private groups aimed for collaboration as you would now be able to see who all have seen the latest updates.
The check under each post indicates how many group members have seen it. This way you can stay updated on the group’s activity.