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.

If connected to a site, the group will send out each blog post as an email to group-site members. this is helpful for sending group-site users news or announcements from the site. Often, group-site use this feature as a sort-of list-serv. When Commons join the group-site, they are kept abreast of what is going on with the group-site news and updates.

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.