WHO
The answer to the question "Who" are people, roles and groups:
classDiagram
Who <|-- Person
Who : +String name
Who <|-- Role
Who <|-- Group
classDiagram
Person "1" --> "many" Role : ActsAs
Role "1" --> "many" Group : MemberOf
Role "1" --> "1" Role : ReportsTo
QPerson
QRole
QGroup