query: SELECT ?item (COUNT(DISTINCT ?person) AS ?members) WHERE {
    ?item p:P31 _:b2.
    _:b2 ps:P31 wd:Q15238777;
      pq:P642 wd:Q2867079.
    OPTIONAL {
      ?person p:P39 _:b3.
      _:b3 pq:P2937 ?item.
    }
  }
  GROUP BY ?item