Suppose if you have a dam-parent group which is applicable to all folders inside DAM, then create a new group called "brandassets-admin" and add your dam-parent group to your new group. Now your new group have all the permissions as your dam-parent group.
Now, go to your dam-parent group and deny permission to the folders which you want to show to only "brandassets-admin". Then go to "brandassets-admin" group and go to permissions tab and allow those specific folders which you want to show to only the new group.
content-authors group is the parent group with access to all content and I have added the brandassets-admin to the same. Lets say I dont want to show We.Retail DAM to this user. I go to the permissions and remove that particular content access .
Similar way you can set permissions for the other groups