When you are a member of a closed group, only the people inside that group will be able to see your comments, posts, and likes. If your friends are not members of that closed group, then your comments/posts/likes will not appear on your friends' news feeds (and neither will your joining of that group). But if you have any friends who have joined that group, then they will be able to see anything you do in the group.
Here is a Facebook article that describes the privacy of Facebook groups. It says the following about closed groups:
Who can see what members post in the group? Only current members
Who can see stories about the group on Facebook (like in News Feed and search)? Only current members