An easy subject this time. To configure subgrid we need to add few parameters to grid initialization code:

<script type="text/javascript">
  $(document).ready(function() {
    $('#jqgOrders').jqGrid({
      //url from wich data should be requested
      url: '/Home/OrdersGridData/',
      //type of data
      datatype: 'json',
      //url access method type
      mtype: 'GET',
      //columns names
      colNames: ['OrderID', 'Customer', 'Employee', 'OrderDate', 'RequiredDate', 'ShippedDate', 'ShipCountry'],
      //columns model
      colModel: [
                  { name: 'OrderID', index: 'OrderID', align: 'left' },
                  { name: 'Customer', index: 'CustomerID', align: 'left' },
                  { name: 'Employee', index: 'EmployeeID', align: 'left' },
                  { name: 'OrderDate', index: 'OrderDate', align: 'left' },
                  { name: 'RequiredDate', index: 'RequiredDate', align: 'left' },
                  { name: 'ShippedDate', index: 'ShippedDate', align: 'left' },
                  { name: 'ShipCountry', index: 'ShipCountry', align: 'left' }
                ],
      //pager for grid
      pager: $('#jqgpOrders'),
      //number of rows per page
      rowNum: 10,
      //initial sorting column
      sortname: 'OrderID',
      //initial sorting direction
      sortorder: 'asc',
      //we want to display total records count
      viewrecords: true,
      //grid width
      width: 'auto',
      //grid height
      height: 'auto',
      //enable subgrid
      subGrid: true,
      //subrid model
      subGridModel: [{
                       //subgrid columns names
                       name: ['ProductName', 'UnitPrice', 'Quantity', 'Discount'],
                       //subgrid columns widths
                       width: [200, 100, 100, 100],
                       //subrig columns aligns
                       align: ['left', 'left', 'left', 'left']
                     }],
      //url from which subgrid data should be requested
      subGridUrl: '/Home/OrdersDetailsSubgridData/'
    });
  });
</script>

Controller action for main grid is pretty standard (you can take a look at corresponding sample in jqGrid basics post). The one for subgrid is slightly different. That's because of  the different data format - subgrid takes only rows collection with cell table inside.

/// <summary>
/// Provides json data for jqGrid subgrid
/// </summary>
/// <param name="id">parent row id</param>
/// <returns>json data</returns>
public ActionResult OrdersDetailsSubgridData(string id)
{
  //Preparing anonymous variable with json data
  var ordersDetails = new
  {
    rows = (from orderDetails in _repository.GetOrderDetails(Convert.ToInt32(id))
            select new
            {
              //table of cells values
              cell = new string[] {
                                    orderDetails.Product.ProductName,
                                    orderDetails.UnitPrice.ToString(),
                                    orderDetails.Quantity.ToString(),
                                    orderDetails.Discount.ToString()
                                  }
            }
           ).ToArray()
  };

  //Returning json data
  return Json(ordersDetails);
}

Subgrid is very limited (no support for paging, sorting, searching and editing). There is a solution which uses some of the subgrid functions of the main grid to create not a subgrid, but another fully functional grid. This solution is well described here and I'm not going to dig into it (it's just a little bit tricky initialization javascript). Let's take a look at standard subgrid we have created:
Of course you can download sample application here.