db={notifier:[{"_id":"66d90957373dd9d5c85d503c","REF_NotifierID":"66d9070f373dd9d5c85d5034","REF_UserID":"66c8fbb1a97fee2ae10a3d81"},{"_id":"66d90a33373dd9d5c85d5050","REF_NotifierID":"66d90700373dd9d5c85d502e","REF_UserID":"66c8fbb1a97fee2ae10a3d81"},{"_id":"66d9ca6be74061bbd0d1996a","REF_NotifierID":"66d90700373dd9d5c85d502e","REF_UserID":"66d9c3ddc1d6fcc235ccc92e"}],users:[{"_id":"66c8fbb1a97fee2ae10a3d81","firstname":"alice","lastname":"smith"},{"_id":"66d9c3ddc1d6fcc235ccc92e","firstname":"bob"}]}

db.notifier.aggregate([{$group:{_id:"$REF_NotifierID",users:{$push:{REF_UserID:"$REF_UserID"}}}},{$lookup:{let:{docs:"$users"},as:"users",pipeline:[{$documents:"$$docs"},{$lookup:{from:"users",localField:"REF_UserID",foreignField:"_id",as:"userData",pipeline:[{$project:{_id:0,firstname:1,lastname:1,REF_UserID:"$_id"}}]}},{$unwind:"$userData"},{$replaceRoot:{newRoot:"$userData"}}]}}])