[{_id:"607536219910ef23e80e0bbe",companyname:"Main Company",sponsor:""},{_id:"607e16760a9d2c16e06bc252",companyname:"Company 1",sponsor:"607536219910ef23e80e0bbe"},{_id:"607e187b0a9d2c16e06bc253",companyname:"Company 2",sponsor:"607e16760a9d2c16e06bc252"},{_id:"607e1f470a9d2c16e06bc254",companyname:"Company 3",sponsor:"607e187b0a9d2c16e06bc253"}]

db.collection.aggregate([{$addFields:{userId:{$toString:"$_id"}}},{$graphLookup:{from:"collection",startWith:"$userId",connectFromField:"userId",connectToField:"sponsor",as:"downline",maxDepth:100,restrictSearchWithMatch:{}}}])