Currently folder access can be edited the Administrator. 

To secure a folder, first go to the 'Document Center' and choose the folder you would like to lock. 

For example, to lock the 'Governance' folder fist the Admin will need to open it:

Then, on the right hand side select 'edit folder'

Here the Admin can lock the folder under 'Visibility' then select 'Limited' - and then choose the Group. Only users who are members of this group can see the documents. 

The folder will now have a Lock icon: