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. Then, click Create Group and Continue. This will lead you to steps where you can choose whether to enable discussion boards; 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.
Creating a group is covered in more detail in this guide.