On the Commons, you can associate a site to a group. This is extends the functionality of both the group and the site. The group may be private, but it can have a public site. Or both can be private.
Professors often use groups to encourage discussion and sites to share resources, student reflections, syllabi, and contact information.
Group sites can be created when a group is initially created, or an existing site can later be associated to a group. Learn how to create a group site.