Código PHP:
<?php require_once('Connections/conexion.php'); ?>
<?php
mysql_select_db($database_conexion, $conexion);
$query_consulta = "SELECT id, nombre, email, grupo FROM tabla Order By grupo";
$consulta = mysql_query($query_consulta, $conexion) or die(mysql_error());
$row_consulta = mysql_fetch_assoc($consulta);
$totalRows_consulta = mysql_num_rows($consulta);
?><!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" />
<title>Documento sin título</title>
</head>
<body>
<table width="500" border="1" style="border-collapse:collapse;" bordercolor="#000000">
<?php do{ ?>
<?php
$grupoant=$grupo;
$grupo=$row_consulta['grupo'];
?>
<?php if($grupoant != $grupo){?>
<tr>
<td colspan="3" align="center" bgcolor="#CCCCCC"><strong><?php echo $row_consulta['grupo'] ?></strong></td>
</tr>
<?php } ?>
<tr>
<th><?php echo $row_consulta['id']; ?></th>
<th><?php echo $row_consulta['nombre'] ?></th>
<th><?php echo $row_consulta['email'] ?></th>
</tr>
<?php } while ($row_consulta = mysql_fetch_assoc($consulta));?>
</table>
</body>
</html>
<?php
mysql_free_result($consulta);
?>