missing groups inside a course (similar to cohorts)

[Hendrik]

im missing the feature to create groups inside a course, to which the students can enroll themself (until a group is full).

to each lesson we also have an exercise lesson, limited to lets say 15 persons. but those exercise groups are on different dates, so the students should assign themself to the group they want to study in. as far as i could see the cohorts feature doesnt offer that kind of functionality