Nextcloud Teams allow people to create their own groups of people/colleagues/friends. Those groups of people can then be used by any other app for sharing purpose.
The app was formerly named circles which is still used as internal identifier a lot.
The Teams app is installed and enabled by default.
You can use the \Psr\Container\ContainerInterface, see dependency injection, to get the \OCA\Circles\CireclesManager class, see our API documentation for its interface.