Database
[{"_id":{"group":"A","city":"New York"},"totalMembers":1},{"_id":{"group":"A","city":"Seattle"},"totalMembers":2},{"_id":{"group":"B","city":"New York"},"totalMembers":1},{"_id":{"group":"B","city":"Los Angeles"},"totalMembers":2}]
Query
db.collection.aggregate([{$group:{_id:"$_id.group",Cities:{$push:{k:"$_id.city",v:"$totalMembers"}}}},{$project:{_id:1,Cities:{$arrayToObject:"$Cities"}}}])