Add a user to a group

        :type group_name: string
        :param group_name: The name of the group

        :type user_name: string
        :param user_name: The to be added to the group.

def add_user_to_group(self, group_name, user_name):
        """
        Add a user to a group

        :type group_name: string
        :param group_name: The name of the group

        :type user_name: string
        :param user_name: The to be added to the group.

        """
        params = {'GroupName' : group_name,
                  'UserName' : user_name}
        return self.get_response('AddUserToGroup', params)