The reason your select doesn't get populated is because the mysqli_query doesn't get a connection supplied.
Change:
$db = mysqli_query($queryusers);
Into:
$db = mysqli_query($db, $queryusers);
Also add the name between (else you get a list with no text):
while ( $d=mysqli_fetch_assoc($db)) {
echo "<option value='{".$d['Name']."}'>".$d['Name']."</option>";
}
Hope this helped you out!