1. Navigate to the Setup tab and choose 'Groups' in the upper right menu.
  2. Choose 'Edit' under the Actions column next to your default Group.
  3. Scroll down and uncheck the 'Public can view' check box.
  4. Scroll down again and click the blue 'Save' button.
  5. Click 'Groups' on the right to go back to your Group list and repeat steps 2 - 5 until all Groups are hidden from public view.

Learn more about creating and organising Groups by visiting this article.