I don't think MC can do that (unless Wer proves me wrong ;). The second grouping level under the Instrument would need to group files based both on [personnel] AND the value of the parent node (instrument); it would have to be defined by an Expression like:
ListGrep([Personnel],[value_of_the_parent_node])&DataType=[ List ]
AFAIK, that's not supported by MC - there's no variable that we can reference that contains the value of the parent node.
I tested using Save() on the parent node definition to save the name of the node/instrument into a global variable, but it does not work; it saves the full [Instruments] field, not just the current one on the tree.
You can try posting a feature request for a special [ParentNode] or [NodePath] variable. Depending on how MC builds the trees internally, this might be trivial to do or extremely complex.