I have seen many posts about how to make things show in your News Feed by selecting "Get Notifications" and "Show in News Feed", but I find that those simply don't work. (I follow many pages since I administrate my office FB page, so maybe that affects it, but I still miss pages that way.)
First, log into Facebook for either method of setting up interest groups or adding to your group. (Note: if you are a page administrator, you must be logged in as your personal account, not your Page.)
First method: This works well if you are on a page and want to add it to an interest group. Click on the little gear/arrow (in blue below) and then add to Interest Lists.