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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def add(self, nickname, email=None, phone_number=None, user_id=None):
 """Add a user to the group. You must provide either the email, phone number, or user_id that uniquely identifies a user. :param str nickname: new name for the user in the group :param str email: email address of the user :param str phone_number: phone number of the user :param str user_id: user_id of the user :return: a membership request :rtype: :class:`MembershipRequest` """

        member = {
            'nickname': nickname,
            'email': email,
            'phone_number': phone_number,
            'user_id': user_id,
        }
        return self.add_multiple(member)