Here’s the code I was looking for:
$values = $_POST['acf'];
$measurement_mail_extra_recipients = $_POST['acf']['field_5f71d4eaaf381'];
if ( $measurement_mail_extra_recipients ) {
$list = array();
foreach( $measurement_mail_extra_recipients as $measurement_mail_extra_recipient ) {
$list[] = $measurement_mail_extra_recipient['field_5f71d55aaf382', $measurement_client];
}
$to = implode(',', $list);
}