First, select the text block at the top of your “news feed”.
This will open the “create post”. From here, select the box below your name:
This will create a drop-down menu that allows you to select where you want to post. In this example, we want to post to a specific group, so select “post in group”
Note the other options below.
Then select the group you would like to post to and click “save”
And now you can post in the selected group feed and only members in that group will see the content posted.