It´s straightforward to view and manage folder permissions so that your users can access ONLY the folders you want them to access.
To see who can access a folder, enter or select the folder and click the Permissions icon in the Sidebar.
You will now see at a list of the users that has access to this folder (including sub-folders). You can change, add, or remove their folder permissions directly from this menu. Choose between View only, + Download, + Upload, or remove.
To add and manage users you can click the MANAGE USERS button. This will take you to Admin > Users where many more user specific actions can be performed.