[{"name":"Wendy","phone":"123","GroupId":1},{"name":"Tom","phone":"234","GroupId":1},{"name":"Sally","phone":"345","GroupId":3},{"name":"Bob","phone":"456","GroupId":3},{"name":"Cortana","phone":"567","GroupId":7}]

db.collection.aggregate([{$group:{_id:"$GroupId",name:{$first:"$name"},phone:{$first:"$phone"},GroupId:{$first:"$GroupId"}}},{$project:{_id:0}}])