The answer is to create another, smaller group and only invite a few people to it. Then share your planbook with both groups, but only give editing access to one of the groups.
For example, lets say my school has a group called "KASA College Prep" with 20 people in it, but I only want to co-edit with 3 people at my school. Here's what I would do:
I could also check to see which groups my planbook is shared with and change the permission levels for any of those groups at any time.