{
        "_id" : ObjectId("557eaf444ba222d545c3dfcc"),
        "guest_id" : ObjectId("5c2aad871dbbe3194ed45023"),
        "parameter" : 354,
        "created" : ISODate("2019-01-01 00:00:05.683Z"),
    }

db.events.aggregate([
      {
          $group:
          {
              _id: "$parameter",
              guestList: {$addToSet: {id: "$guest_id", created: "$created"} },
              count: {$sum: 1}
          }
      }
  ]);

guestList: {$addToSet: {id: "$guest_id"} },