It would be helpful if they offered a "degree" of some kind for completing groups of classes. Mixing it with the concept of gamification like Khan Academy does might also help with participation.
They do.
EdX - xSeries [1]
Coursera - Specializations [2]
Udacity - Nanodegrees [3]
Obviously, no one agrees on what to call these groups. Worse, the length, cost and schedules are equally disparate which isn't a problem specific to groups, but the state of MOOCs in general.
One group offering might be 7 classes at $50 each, with each class covering 8 weeks with an estimated effort of 8-10 hours per week. Another is 9 classes at $50 each, with each class covering 4 weeks with an estimated effort of as little as 3-4 hours per week. One group can be completed in as little as three months, another requires a minimum of 3 years.
1: https://www.edx.org/xseries
2: https://www.coursera.org/specializations?utm_medium=topnav
I am suspicious of this claim because I recently did an analysis of this data: http://minimaxir.com/2014/07/online-class-charts/
The average amount of students who complete classes that they've registered for is about 3.4%, much lower than they indicate: http://minimaxir.com/img/online-class-charts/class-attendanc...