When you visit the group page that you are interested in joining...
on the right hand side a button is displayed with the text "Join Group".
With this component, (just download and install/activate the component in your admin panel) all the groups you join will display in your newsfeed.
AS AN ADMIN, YOU WILL NOT BE ABLE TO SEE THE GROUP POST IN YOUR NEWSFEED.
Just create a general user account, and log in with that account to see your site as it would appear to your users. The general user account must join the groups that you want displayed in the general user's newsfeed.