query: SELECT ?item (COUNT(DISTINCT ?person) AS ?members) WHERE { 
    ?item p:P31 [ ps:P31 wd:Q15238777 ; pq:P642 wd:Q11268 ] .
    OPTIONAL { ?person p:P39 [ps:P39 wd:Q4416090; pq:P2937 ?item] }
  }
  GROUP BY ?item