Each folder in the briefcase, including the top level, has its own set of permissions set in the "Security" settings. The security settings are accessed by clicking the lock icon when viewing a folder logged in as someone with the "Full" permission for that
folder (usually the folder creator). Anyone with the "Modify" permission for that folder can create new folders. Permissions do not cascade down (setting a permission on a parent folder does not affect the permissions of the children).
I hope this helps!