Assigning students to classes

Is it me or it the method to assign a student to class/roll group rather well hidden. Two of us have been looking for it today. I can go to user admin - manage users and I can see the student’s roll group but when I edit details I can change their house but not their roll group.
I can look at form groups but there is nowhere to add a student to the form group.
I can’t see anything on the permissions page that I might have missed.
So now I feel stupid. How do I do it?

Sorted!

Andy, glad you got this sorted. I believe you found People > Students > Student Enrolment, which is the final step in making a student a student.