[{name:"a",title:{title:"b",order:1},group:"B"},{name:"b",title:{title:"b",order:1},group:"B"},{name:"c",title:{title:"c",order:2},group:"B"}]

db.collection.aggregate([{"$group":{"_id":"$title.title","items":{"$addToSet":{"name":"$name","group":"$group"}}}}])