We can put the summary calculation on the header group.
To do this just put the summary field in the groupText. The field is enclosed in {}.

If you do not want to show the group footer set groupSummary values to false
<?php 
require_once '../../../../php/demo/tabs.php';
?>
<!DOCTYPE html>
<html>
  <head>
    <title>jqGrid PHP Demo</title>
    <link rel="stylesheet" type="text/css" media="screen" href="../../../../css/jquery-ui.css" />
    <link rel="stylesheet" type="text/css" media="screen" href="../../../../css/trirand/ui.jqgrid.css" />
    <link rel="stylesheet" type="text/css" media="screen" href="../../../../css/ui.multiselect.css" />
    <style type="text">
        html, body {
        margin: 0;            /* Remove body margin/padding */
        padding: 0;
        overflow: hidden;    /* Remove scroll bars on browser window */
        font-size: 75%;
        }
        
    </style>
    <script src="../../../../js/jquery.min.js" type="text/javascript"></script>
    <script src="../../../../js/trirand/i18n/grid.locale-en.js" type="text/javascript"></script>
         <script src="../../../../js/trirand/jquery.jqGrid.min.js" type="text/javascript"></script> <script type="text/javascript">         
    $.jgrid.no_legacy_api = true;
    $.jgrid.useJSON = true;
    $.jgrid.defaults.width = "700";
    </script>
     
    <script src="../../../../js/jquery-ui.min.js" type="text/javascript"></script>
  </head>
  <body>
      <div>
          <?php include ("grid.php");?>
      </div>
      <br/>
      <?php tabs(array("grid.php"));?>
   </body>
</html>
grid.php.
<?php
require_once '../../jq-config.php';
// include the jqGrid Class
require_once ABSPATH."php/PHPSuito/jqGrid.php";
// include the driver class
require_once ABSPATH."php/PHPSuito/DBdrivers/jqGridPdo.php";
// Connection to the server
$conn = new PDO(DB_DSN,DB_USER,DB_PASSWORD);
// Tell the db that we use utf-8
$conn->query("SET NAMES utf8");

// Create the jqGrid instance
$grid = new jqGridRender($conn);
// Write the SQL Query
$grid->SelectCommand = 'SELECT OrderID, OrderDate, CustomerID, EmployeeID, Freight, ShipName FROM orders';
// set the ouput format to json
$grid->dataType = 'json';
// Let the grid create the model from SQL query
$grid->setColModel();
// Set the url from where we obtain the data
$grid->setUrl('grid.php');
// Set alternate background using altRows property
$grid->setGridOptions(array(
    "rowNum"=>10,
    "sortname"=>"OrderID",
    "rowList"=>array(10,20,50),
    "height"=>'auto',
    "footerrow"=>true,
    "userDataOnFooter"=>true,
    "grouping"=>true,
    "groupingView"=>array(
        "groupField" => array('CustomerID', 'EmployeeID'),
        "groupColumnShow" => array(true, true),
        "groupText" =>array('<b> Client:{0}</b> Sum Freight: {Freight}', '<b>Employee: {0}</b> Sum Freight: {Freight}'),
        "groupSummary" => array(false, false)
    ) 
    ));
// Change some property of the field(s)
$grid->setColProperty("OrderID", array("label"=>"ID", "width"=>60));
$grid->setColProperty("OrderDate", array(
    "formatter"=>"date",
    "formatoptions"=>array("srcformat"=>"Y-m-d H:i:s","newformat"=>"m/d/Y")
    )
);
// Add a summary property to the Freight Column
$grid->setColProperty("Freight", array("summaryType"=>"sum", "summaryTpl"=>"{0}", "formatter"=>"number"));
$summaryrows = array("Freight"=>array("Freight"=>"SUM"));
// Enjoy
$grid->renderGrid('#grid','#pager',true, $summaryrows, null, true,true);