SonataUserBundle/Resources/config/doctrine/User.orm.xml

<?xml version="1.0" encoding="UTF-8"?>
<doctrine-mapping xmlns="http://doctrine-project.org/schemas/orm/doctrine-mapping"
                  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
                  xsi:schemaLocation="http://doctrine-project.org/schemas/orm/doctrine-mapping
                  http://doctrine-project.org/schemas/orm/doctrine-mapping.xsd">

    <entity name="Application\Sonata\UserBundle\Entity\User" table="fos_user_user">

        <id name="id" column="id" type="integer">
            <generator strategy="AUTO" />
        </id>

        <many-to-many target-entity="Group" field="groups">
            <join-table name="fos_user_user_group">
                <join-columns>
                    <join-column name="user_id" referenced-column-name="id" />
                </join-columns>
                <inverse-join-columns>
                    <join-column name="group_id" referenced-column-name="id" />
                </inverse-join-columns>
            </join-table>
        </many-to-many>

    </entity>

</doctrine-mapping>