Esta seria a minha solução, embora não seja nada elegante
<?php
$dbc = new MySQLI(DBHOST,DBUSER,DBPASS,DB);
$result = $dbc->query("
SELECT
p.Group as 'group',
GROUP_CONCAT(name) as names
FROM prueba p
GROUP BY p.Group
");
?>
<table>
<tr>
<th>Group</th>
<th>Name</th>
</tr>
<?php while($row = $result->fetch_assoc()){
$names = split(",",$row["names"]);
?>
<tr>
<td><?php echo $row["group"] ?> </td>
<td><?php echo $names[0]; array_shift($names) ?></td>
</tr>
<?php foreach( $names as $name){ ?>
<tr>
<td></td>
<td><?php echo $name ?></td>
</tr>
<?php } ?>
<?php } ?>
</table>
