Astarte supports creating groups of devices in a realm.
Groups are currently useful mainly to provide access control, combining Astarte's path based authorization with the fact that devices can be queried with a group URL. This makes it possible to emit tokens allowing a user to operate only on devices that belong to a specific group.
Keep in mind a group is existing as long as there's at least one device in it. Once the last device is removed from the group, the group does not exist anymore, since groups are a tag (or label) of devices.