newsupernova/dev/tests/integration/testsuite/Magento/Customer/etc/extension_attributes.xml

<?xml version="1.0"?>
<!--
/**
 * Copyright © Magento, Inc. All rights reserved.
 * See COPYING.txt for license details.
 */
-->
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Api/etc/extension_attributes.xsd">
    <extension_attributes for="Magento\Customer\Api\Data\AddressInterface">
        <attribute code="test_customer_group_id" type="int">
            <join reference_table="customer_entity"
                  reference_field="entity_id"
                  join_on_field="parent_id"
                    >
                <field>group_id</field>
            </join>
        </attribute>
    </extension_attributes>
</config>