It is now possible to group users into teams and see progress reports for each team in task lists and analytics views (see the new "Teams" slider). Organization administrators with manage teams permission may create, update and delete teams. Users with Admin role automatically get this permission.
The relationship between users and teams is many-to-many - a user may be added into several teams at the same time.
At this moment the usage of teams is limited to the new "Teams" slider, but in the future, teams will be used to provide improved analytics like team velocity reports.