I think it would be beneficial if teachers could define custom sharing groups when adding to Commons. For example, teachers can share with any Sub-Account they are teacher for a course in, and we have teachers with courses at multiple schools, allowing them to see multiple locations to share with, which is good. However, because of how our data comes out of our SIS our teachers don't belong to a defined Sub-Accounts like HS English or MS Math, and instead they have all or none sharing with an entire building. If teachers could define individuals as part of a sharing group it would ease the administrative side of defining additional sub-accounts, and then creating and placing teachers into additional courses for classification purposes.
I'm excited to say we are working on the ability to share resources to a custom group of users that has been created by an admin. I hope that this might be a solution to the concern you share above. As we roll out this feature later this year we would love to hear feedback on how it works for you.