db={"users":[{_id:"1",name:"Alex"},{_id:"2",name:"Blex"},{_id:"3",name:"Clex"},{_id:"4",name:"Dlex"},{_id:"5",name:"Elex"},{_id:"6",name:"Flex"},],"events":[{_id:"1",name:"Upgrade to Premium",users:["1","4","6"]}]}

db.events.aggregate([{"$lookup":{"from":"users","let":{user_ids:"$users"},pipeline:[{$match:{$expr:{$not:{$in:["$_id","$$user_ids"]}}}}],"as":"AbsentUsers"}}])