Mongoose sort the aggregated result

The results returned from the aggregation pipeline are just plain objects. So you do the sorting as a pipeline stage, not as a separate operation:

Recommend.aggregate(
    [
        // Grouping pipeline
        { "$group": { 
            "_id": '$roomId', 
            "recommendCount": { "$sum": 1 }
        }},
        // Sorting pipeline
        { "$sort": { "recommendCount": -1 } },
        // Optionally limit results
        { "$limit": 5 }
    ],
    function(err,result) {

       // Result is an array of documents
    }
);

So there are various pipeline operators that can be used to $group or $sort or $limit and other things as well. These can be presented in any order, and as many times as required. Just understanding that one “pipeline” stage flows results into the next to act on.