Every user can ask to join public Discussion, whether they are open or with controled access.
You can ask to join the Discussion in two ways:
1. left navigation menu > All discussion > find discussion you want to join and click on the green door in section Actions of the listing
2. access the desired Discussion and on the right side of the Discussion page view click on the green door to join the discussion.
Once on the Discussion preview, you gain access to the Start new topic green button.That button will trigger the Topic submission form.