[{"_id":1,"username":"abcd","createdDate":ISODate("2016-06-03T08:52:32.434Z")},{"_id":2,"username":"abcd","createdDate":ISODate("2016-05-03T09:52:32.434Z")},{"_id":3,"username":"abcd","createdDate":ISODate("2016-04-03T10:52:32.434Z")},{"_id":4,"username":"xyz","createdDate":ISODate("2016-03-03T10:52:32.434Z")},{"_id":5,"username":"xyz","createdDate":ISODate("2016-02-03T10:52:32.434Z")},{"_id":6,"username":"zzz","createdDate":ISODate("2016-01-03T10:52:32.434Z")}]

db.collection.aggregate([{"$match":{"username":{"$ne":"zzz"}}},{"$group":{_id:"$username","count":{"$sum":1},date:{$max:"$createdDate"}}},{$project:{username:"$_id",total:"$count",createdDate:"$date"}}])