[{_id:13123123123,title:"TitleofObject",series:"SeriesName",},{_id:1312312,title:"TitleofObject3",series:"SeriesName",},{_id:11,title:"TitleofObject2",series:"SeriesName",}]

db.collection.aggregate([{"$group":{"_id":"$series","sameSeries":{$push:{"title":"$title","_id":"$_id"}}}}])