Use the ICISite interface SetPermission method to add folder permissions or to change existing permissions.
Signature:
HRESULT SetPermission (
[in]IPermission *pPermission,
[in, optional] VARIANT_BOOL bRemoveOtherPermissions);
Optional Parameter
If you want to set permissions on a folder for a user and remove all other permissions, pass TRUE to the second parameter of setPermission(). This will REMOVE all other permissions from a folder (that is, break the inheritance).
For example,
set perm = Site.GetBlankPermission( "/usr/foo", "foo" )
perm.FileDelete = True
perm.FileUpload = True
perm.FileDownload = False
Site.SetPermission( perm, True )