A closed group means you can only view the content of the group if you are a member. If you are not a member, you will be shown the following message:

'This is a closed group, so content is only available to group members."

The group settings can only be edited by managers in the group settings. 

In our initial version of the Groups feature, all groups are closed by default. More enhancements are on the way to allow this setting to be edited in future. We welcome your feedback on the Groups feature, and would love to hear what you are finding useful and which enhancements you feel are the most urgent.