Unfortunately people only enter the groups and do nothing after that. When you are in a group you just see a list of members.
But, there is a ‘Show’ button on the right top of any group page. You can choose ‘Updates’ there and see all posts people have already made. Not enough to swear by – кот наплакал ;(
Share your ideas! Otherwise for what those groups are… I think reading information in a language you learn could help with improving it. If there would be a long list of posts you could choose just perfect one for your intention, I am sure. But when there are no updates, you can do nothing either..