Facebook Groups now show users who has viewed their posts.

The new feature is gradually getting rolled out like all of Facebook's updates.Facebook has started rolling out an update to Groups that allows users to view who has seen each post.  Here's the official description of the feature from the Facebook Help Center (Group basics and Group features):

How do I know who's seen each post in a group? The check under each post indicates how many group members have seen it. This way you can stay updated on the group's activity. Hover over the check to get an idea of who's seen it and when. Anyone who can view the group post will see the check. What does it mean if a post in a group or message on Facebook is marked as "seen?" Facebook Messages and posts in groups are marked as "seen" after your friends or group members have seen them. Just keep in mind that if people see a group post or message, it doesn't always mean they had the chance to read it carefully.

This "seen" concept is more useful in Messages than in Groups. The Messages feature became available a few months ago, first with Messenger 1.7 and then expanded to all Facebook Messages.

Overall, users are starting to say they find "seen" particularly useful on mobile. With Groups, Facebook offers a pitch via an example: a soccer team using the new Groups "seen" feature to make sure everyone knows when the next practice starts. Many Facebook members, who are part of Groups with a large number of people, and messages, may not want to inform everyone else they checked the Group. One shortcoming, this feature cannot be turned off. Privacy advocates are likely already trying to figure out how they can legally punish Facebook for skipping out on such an option.