db={"users":[{_id:ObjectId("5a934e000102030405000001"),name:"John Doe"},{_id:ObjectId("5a934e000102030405000002"),name:"Jane Roe"}],groups:[{_id:100,name:"A Group",members:[ObjectId("5a934e000102030405000001"),ObjectId("5a934e000102030405000002")]}]}

db.users.aggregate([{$lookup:{"from":"groups","localField":"_id","foreignField":"members","as":"membersInfo"}},{$unwind:"$membersInfo"},{$project:{"_id":{$cond:{"if":{$in:["$_id","$membersInfo.members"]},"then":"$_id"}},"name":1,"_group":"$membersInfo._id"}}])