{
   "_id" : ObjectId("5a377b895a2840568b1034d5"),
   "reportId" : 1234,
   "quantity" : 346
},
{
  "_id" : ObjectId("5a377b935a2840568b1034d6"),
  "reportId" : 1234,
  "users" : 96
},
{
  "_id" : ObjectId("5a377ba35a2840568b1034d7"),
  "reportId" : 2345,
  "quantity" : 566
},
{
  "_id" : ObjectId("5a377bac5a2840568b1034d8"),
  "reportId" : 2345,
  "users" : 66
}

{'reportId': 1234, 'total':quantity * users}
and so on
where total in the multiplication of users and quantity

db.test1.aggregate([
{'$group':
  { '_id':'$reportId',
    'total':{'$multiply':['$users','$quantity']}
  }
}])

assert: command failed: {
    "ok" : 0,
    "errmsg" : "The $multiply accumulator is a unary operator",
    "code" : 40237,
    "codeName" : "Location40237"
} : aggregate failed

db.test1.aggregate([ {
'$group':
         { '_id':'$reportId',
           'document':{'$push':{'reportId':'$reportId',
                       'quantity':'$quantity','users':'$users'}}}},
{'$unwind':'$document'}])

{ "_id" : 2345, "document" : { "reportId" : 2345, "quantity" : 566 } }
{ "_id" : 2345, "document" : { "reportId" : 2345, "users" : 66 } }
{ "_id" : 1234, "document" : { "reportId" : 1234, "quantity" : 346 } }
{ "_id" : 1234, "document" : { "reportId" : 1234, "users" : 96 } }