[{_id:123,example_document:true,people:[{name:"John",age:18},{name:"Clint",age:20}]}]

db.collection.aggregate([{$addFields:{concat_names:{$reduce:{input:"$people",initialValue:"",in:{$concat:["$$value",{$cond:[{$eq:[{"$strLenCP":"$$value"},0]},"",", "]},"$$this.name"]}}}}}])