After you’ve logged in to the Commons, click on the Groups tab; then, select the Create a Group button next to the group directory. Enter a group name and group description; you can also write some news about your new group. Then, click Create Group and Continue. You will next be able to choose whether to enable discussion forums; select whether your group will be public, private, or hidden; determine the member roles in your group; create a group avatar; and invite members to join your group.
This is covered in more detail here.