Groups are really easy to set up. Login as a Teacher or Administrator and click on the My Groups menu link to take you to your groups page.
Groups are private to each individual teacher and are a way to group any users across the school together. A user does not need to be part of a class that you teach in order to be added to a group. For example, a head of department might want to create a group containing all of the teachers within their department. This will make it much easier to share documents through the quick share feature and to set assignments to groups of users who are not in a class together.
Once your group is created, to add users to the group, start typing their user name into the search box. This will search all of the users across your school; students, teachers and administrators. Anyone with an Office 365 account who is not hidden will be searched.