How do I create joinable / un-joinable groups?
The Site Editor / Manage Groups tool contains a feature named "Create Joinable Set" that automatically creates joinable/un-joinable groups.
Groups can have separate Assignments, Forums, Topics, Tests/Quizzes, Announcements, Lesson pages, Email or Resources.
Students join/unjoin groups via their Site Editor tool. See: How do I join / unjoin a group? (Student)