add_participant.xsd

<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">

    <xs:include schemaLocation="error.xsd"/>

    <xs:complexType name="Participant">
        <xs:attribute name="number" use="optional"/> <!-- If number is specified conversation_id will be ignored -->
        <xs:attribute name="conversation_id" use="optional"/>
    </xs:complexType>

    <xs:complexType name="AddToConference">
        <xs:all>
            <xs:element name="participants" minOccurs="0">
                <xs:complexType>
                    <xs:sequence>
                        <xs:element name="participant" minOccurs="0" maxOccurs="unbounded" type="Participant"/>
                    </xs:sequence>
                    <xs:attribute name="conference_id" type="xs:string" use="required"/>
                </xs:complexType>
            </xs:element>
        </xs:all>
    </xs:complexType>

    <!-- in -->
    <xs:element name="in">
        <xs:complexType>
            <xs:all>
                <xs:element name="request">
                    <xs:complexType>
                        <xs:sequence>
                            <xs:element name="add_to_conference" type="AddToConference"/>
                        </xs:sequence>
                    </xs:complexType>
                </xs:element>
            </xs:all>
        </xs:complexType>
    </xs:element>
    <!-- out -->
    <xs:element name="out">
        <xs:complexType>
            <xs:choice>
                <xs:element name="response">
                    <xs:complexType>
                        <xs:sequence>
                            <xs:element name="ok"/>
                        </xs:sequence>
                    </xs:complexType>
                </xs:element>
                <xs:element name="error" type="errorType"/>
            </xs:choice>
        </xs:complexType>
    </xs:element>
</xs:schema>