This feature is available in all editions.
Members are not explicitly assigned to a team.
Stories are assigned to a team during Release Planning or Sprint/Iteration Planning. When members are assigned to any of those stories, they are effectively assigned to the team. This allows members to do work for a team without the overhead of managing team membership, as people frequently “help out” other teams, move from one team to another, etc.
Here's How it Works
A story is assigned to a team > A member is assigned to the story > The member becomes a part of the team
A story is assigned to a sprint/iteration > The story is assigned to a team > A member is assigned to the story > The member becomes a part of the team