In order to create a new user the arguments for acf_form would need to look something like this
$args = array(
'post_id' => 'new_user',
'submit_value' => 'Register',
'return' => get_permalink(54),
'uploader' => 'wp',
'field_groups' => array(142)
);
function user_register_pre_save( $post_id ) {
// check if this is to be a new post
if( $post_id != 'new_user' ) {
return $post_id;
}
// rest of your function here...
and then the beginning of your pre_save_post filter would look something like this