When we first introduced the OneNote Binders, amongst the immediate requests was for group collaboration spaces. For the first year, we had the teachers request them through our IT request system and manually created them and assigned the permissions. But, given the convenience of OneNote, we knew we wanted a dynamic solution... and so we rolled out a webapp that would create groups of any size or shape!
The teacher goes to the webapp and decides on a name for the Project and then assigns students to groups. Some teachers will create only one group and then they have a section that everyone can contribute to. Groups can be of any size and students can be in more than one group. The groups can also change as needed, so if you need to shuffle a person to a different group, once you do the permissions are changed to reflect their new position.
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Unlike the regular student sections, the group sections are not available to the student's parents since parents would also see the work of other students.
Teachers have been very creative in using these collaborative spaces... some have used them for discussion boards, for script writing, for Minecraft project development (since screen clipping and sketches work so well in a pen-based tablet environment), math exam review. Really, whenever you want students to work together. And since it's so easy to copy content back into their personal sections, you can still tease out what they're contributing after the group work has complete.
While dynamic groups aren't yet (January 2015) a part of Microsoft's version of the OneNote Class Notebook given the success we've had (and the general clamouring for collaboration) I'd expect to see it appear relatively soon.