Nice explanation Stuart. Thanks 
Can this scenario be extended a bit in the following way:
A school has various Course Categories and you only want to give certain students access to certain course categories. Example: students in the "Free" group only have access to the courses in categories A, B and C while the students in the "Paying" group have access to the courses in categories A, B, C and categories D, E and F. In other words, "Free" students won't have access to categories D, E and F.