[{gameId:"0001A",score:40,name:"Bob",city:"London"},{gameId:"0001A",score:160,name:"Bob",city:"London"},{gameId:"0001A",score:100,name:"Anna",city:"London"},{gameId:"0001A",score:150,name:"Sally",city:"London"},{gameId:"0001A",score:50,name:"Sue",city:"London"},{gameId:"0001A",score:200,name:"Tim",city:"Barcelona"},{gameId:"0001A",score:150,name:"Tina",city:"Barcelona"},{gameId:"0001A",score:50,name:"Amir",city:"Barcelona"},]

db.collection.aggregate([{"$group":{"_id":{game:"$gameId",city:"$city"},"totalScore":{"$sum":"$score"},"people":{"$addToSet":"$name"}}},{"$addFields":{"peopleCount":{"$size":"$people"}}},{"$group":{"_id":"$_id.game","cities":{"$push":{"$arrayToObject":[[{k:"$_id.city",v:{people:"$people",totalScore:"$totalScore",peopleCount:"$peopleCount"}}]]}}}}])