[{all_users:[{id:1,name:"A"},{id:2,name:"B"},{id:3,name:"C"}],selected_users:[{id:1,name:"A"},{id:2,name:"B"}]}]

db.collection.aggregate([{$project:{all_users:{$map:{input:"$all_users",in:{$mergeObjects:["$$this",{selected:{$cond:[{$in:["$$this.id","$selected_users.id"]},"yes","no"]}}]}}}}}])