db={"task":[{"username":"john","taskId":"001","date":"2020-02-18T20:14:19.000Z",},{"username":"david","taskId":"001","date":"2020-02-18T21:48:19.000Z",},{"username":"john","taskId":"002","date":"2020-02-15T20:20:32.000Z",}]}

db.task.aggregate([{$sort:{taskId:1,date:-1}},{$group:{_id:"$taskId",users:{$push:{username:"$username",date:"$date"}}}},{$facet:{tasks:[{$sort:{_id:1}},{$project:{_id:0,taskid:"$_id",users:1}}]}}])