A team is a generic construct for grouping a set of people. People are members of a team. Teams can be empty or contain one or more members.
Teams can optionally exist in a hierarchal tree structure. In this graph, teams have a single parent team and optionally one or multiple child teams. A person can thus be considered a member of a team in two different ways:
directly by being an explicit member of a team, or
indirectly by being a member of a child team.