To enable this within a Topic, go to the Group Page
  1. Select the Topics Tab > + Create Topic
  2. Title the Topic
  3. Select the Layout and add a Cover Photo if needed
  4. Click Save and Add Materials
  5. Drag the Discussion Content Type from the left column into the right column onto the Page where you would like for this to display
All Users that have access to the Group Page can post to the Discussion, this will be seen by all users of the Group within the Topics Page