db={Users:[{_id:ObjectId("5a934e000102030405000000"),first_name:"Sam",last_name:"Jones",email:"samjones@gmail.com",inbox:[{participant:ObjectId("5a934e000102030405000001"),added_at:"12:00:00 09/21/2021"},{participant:ObjectId("5a934e000102030405000002"),added_at:"12:00:00 11/21/2022"},]},{_id:ObjectId("5a934e000102030405000001"),first_name:"John",last_name:"Doe",email:"johndoe@gmail.com",inbox:[]},{_id:ObjectId("5a934e000102030405000002"),first_name:"Jack",last_name:"Smith",email:"jacksmith@gmail.com",inbox:[]}]}

db.Users.aggregate([{$lookup:{from:"Users",localField:"inbox.participant",foreignField:"_id",as:"participants",pipeline:[{$project:{first_name:1,last_name:1,_id:1}}]}},{$set:{participants:{$map:{input:"$participants",in:{$mergeObjects:["$$this",{added_at:{$arrayElemAt:["$inbox.added_at",{$indexOfArray:["$inbox.participant","$$this._id"]}]}}]}}}}}])