Doctrine\MongoDB\Aggregation\Stage::group PHP Method

Groups documents by some specified expression and outputs to the next stage a document for each distinct grouping.

See also: http://docs.mongodb.org/manual/reference/operator/aggregation/group/

public group ( ) : Group
return	Doctrine\MongoDB\Aggregation\Stage\Group

    public function group()
    {
        return $this->builder->group();
    }

Stage