    def post(self, group_name):
        """POST adds users to a group"""
        group = self.find_group(group_name)
        data = self.get_json_body()
        self._check_group_model(data)
        if 'users' not in data:
            raise web.HTTPError(400, "Must specify users to add")
        self.log.info("Adding %i users to group %s", len(data['users']), group_name)
        self.log.debug("Adding: %s", data['users'])
        for user in self._usernames_to_users(data['users']):
            if user not in group.users:
                group.users.append(user)
            else:
                self.log.warning("User %s already in group %s", user.name, group_name)
        self.db.commit()
        self.write(json.dumps(self.group_model(group)))
POST adds users to a group
jupyterhub/apihandlers/groups.py