It was signalled at the GAFE Summit in Sydney in April, but today is the first time that I have seen how you can control the on-sharing of your Google docs.
Click on Share at the top right of your screen, and this window opens:
Select Advanced, and you get this new window:
Just above the blue 'Done' you can Change who has permission to add people:
Why does this matter? Some information in the education context is private to a certain degree and should not be shared on beyond the initial group. This change will make it much easier for schools to fulfil their obligations without someone accidentally sharing confidential information in the wrong circle.