Publicly viewable groups
When creating a new group, a Staff Member also gets the option to make a group Publicly Viewable. By setting a group to be Publicly Viewable you are allowing anyone, both within and outside Mahara, to access the group, including any forums that may exist. People outside the site can only view the group, not take part in discussions.
As you saw, the option to make your group Publicly Viewable appears on the Create Group page when you are setting up your group.
A good example of a time when you might want to make your group Publicly Viewable is when you want users to join your site and take part in discussion. A potential user may find your group via a search engine and discover a forum topic. On realizing that they want to post a response, the user will then need to sign up to join your Mahara. This is great for Mahara sites and groups that reach out into the public domain but is not recommended for those who want their site to be more of a closed "walled garden" for...