[{"_id":1,"Users":[{"name":"John Davies","age":NumberLong(35),"place_of_birth":"France"},{"name":"Edward Jones","age":"blank","place_of_birth":"Spain"},{"name":"Daniel Rhys","age":NumberLong(63),"place_of_birth":"France"},{"name":"Evan Williams","age":NumberLong(21),"place_of_birth":"France"},{"name":"John Davies","age":NumberLong(35),"place_of_birth":"France"}]},{"_id":2,"Users":[{"name":"John Davies","age":NumberLong(35),"place_of_birth":"Italy"},{"name":"Edward Jones","age":"blank","place_of_birth":"Spain"},{"name":"Daniel Rhys","age":NumberLong(63),"place_of_birth":"Italy"},{"name":"Evan Williams","age":NumberLong(61),"place_of_birth":"France"},{"name":"John Davies","age":NumberLong(21),"place_of_birth":"France"}]}]

db.collection.aggregate([{"$unwind":"$Users"},{"$group":{"_id":"$Users.place_of_birth","people":{"$addToSet":{"Name":"$Users.name","Age":"$Users.age","Place of birth":"$Users.place_of_birth"}}}},{"$project":{"_id":1,"Place of birth":"$_id.place_of_birth","total":{"$size":"$people"}}}])