var aggregations = new AggregationDictionary();

var nestedAgg = new AggregationContainer
{
  Nested = new NestedAggregation("some_name1")
  {
    Path = "users",
    Aggregations = new TermsAggregation("some_name2")
    {
       Field = "users.name.keyword",
       Size = 100,
       Order = new List<TermsOrder> { new TermsOrder() { Key = "_term", Order = SortOrder.Descending } }
    }
};

aggregations[key] = nestedAgg;

searchRequest.Aggregations = aggregations;

{
  "aggs": {
     "some_name1": {
         "nested": {
            "path": "users"
          }
     }
}