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

db.collection.aggregate([{$match:{sponsor:""}},{$graphLookup:{from:"collection",startWith:"$_id",connectFromField:"_id",connectToField:"sponsor",depthField:"level",as:"downline"}},{$unwind:{path:"$downline",preserveNullAndEmptyArrays:true}},{$sort:{"downline.level":-1}},{$group:{_id:"$_id",sponsor:{$first:"$sponsor"},companyname:{$first:"$companyname"},downline:{$push:"$downline"}}},{$addFields:{downline:{$reduce:{input:"$downline",initialValue:{level:-1,presentChild:[],prevChild:[]},in:{$let:{vars:{prev:{$cond:[{$eq:["$$value.level","$$this.level"]},"$$value.prevChild","$$value.presentChild"]},current:{$cond:[{$eq:["$$value.level","$$this.level"]},"$$value.presentChild",[]]}},in:{level:"$$this.level",prevChild:"$$prev",presentChild:{$concatArrays:["$$current",[{$mergeObjects:["$$this",{downline:{$filter:{input:"$$prev",as:"e",cond:{$eq:["$$e.sponsor","$$this._id"]}}}}]}]]}}}}}}}},{$addFields:{downline:"$downline.presentChild"}}])