How to secure by a specific manager's organization?

Summary:

Need to be able to secure 1 manager's entire organization.

Content (required):

We currently secure by business unit and department by Person Security Profiles and Areas of Responsibility. Request is to secure by a specific manager's organization. I can't use Department because it spans more than is needed. What other ways can we secure???

Version (include the version you are using, if applicable):

21D

Code Snippet (add any code snippets that support your topic, if applicable):