PREFIX foaf: <http://xmlns.com/foaf/0.1/>
PREFIX dc: <http://purl.org/dc/elements/1.1/>

SELECT ?who ?g ?mbox
FROM <http://my.example/publishers>
FROM NAMED <http://my.example/john>
FROM NAMED <http://my.example/susan>
WHERE { ?g dc:publisher ?who .
        GRAPH ?g { ?x foaf:mbox ?mbox }
}