def add_customers(self, service_desk_id, list_of_usernames):
        """
        Adds one or more existing customers to the given service desk.
        If you need to create a customer, see Create customer method.

        Administer project permission is required, or agents if public signups
        and invites are enabled for the Service Desk project.

        :param service_desk_id: str
        :param list_of_usernames: list
        :return: the customers added to the service desk
        """
        url = 'rest/servicedeskapi/servicedesk/{}/customer'.format(service_desk_id)
        data = {'usernames': list_of_usernames}

        return self.post(url, headers=self.experimental_headers, data=data)