Might add that when using yaml (which returns an array instead of a structure object), you cannot use $structure->add()
(and that method would require a structure item as param, not an array which you pass to it).
So
$structure = page('register')->content()->subscribers()->yaml();
foreach ($csv as $user) {
$structure[] = [
'email' => $user['Email'],
'name' => $user['Name'],
'orderID' => $user['OrderID'],
'datesubscribed' => $user['Paid'],
];
}
$newPage = $parent->update([
'subscribers' => Data::encode($structure, 'yaml'),
]);