When two or more people work together we call this an organization. The Organization could be a School, or a Team for example.
If you are using the platform personally you don't have to have an organization. But the moment you need to group students or teachers you must create an Organization.
The Group is used to group "students" and "instructors". A group could have any number of "students" and "instructors". People in the group could have different level of access and could be in different subscriptions. The "instructors" could have access to "School Teacher Subscription" while the students could have access to "School Student Subscriptions".
Groups have a starting date, groups have a schedule, groups have location. In is the the Course. It is the Group. People participating in one Group could move through many courses and tutorials and materials a in an year.
Groups are part of an organization. If the organization is a school than the groups could be the different classes in the school. Or if the organization is a library, you could use the groups to represent the different clubs in this library. Groups should be specific. There is no "Science Fair Group" in the library. The name should be "2019 Jan-July, Sunday 17:00, Science Fair Group". This means that people in these groups are gathering from Jan to July 2019 and are working on Sunday, 17:00 and are preparing for the "Science Fair".
In one Team there could be many groups. For robotics competitions there could be a "FLL 2018 Team4453 Programming Group" and "FLL 2018 Robot Construction Group" and "FLL 2018 Presentation Group" and "FLL 2018 The Whole Team Group". In this way different assignments could be given to each group and the progress for the Group could be tracked.