multiple email recipients

That’s one way of doing it, or you can locate this chunk of code:

if ($scf_options['scf_mail_function'] === 1) {
        mail($recipient, $topic, $fullmsg, $headers);
        if ($carbon == 1) mail($email, $topic, $fullmsg, $headers);
} else {
        wp_mail($recipient, $topic, $fullmsg, $headers);
        if ($carbon == 1) wp_mail($email, $topic, $fullmsg, $headers);
}

..as you can see, there is a conditional statement that determines if email is sent using mail() or wp_mail(). Adding more addresses would be done like so:

if ($scf_options['scf_mail_function'] === 1) {
        mail($recipient, $topic, $fullmsg, $headers);
        mail('another@recipient.com', $topic, $fullmsg, $headers);
        mail('and-another@recipient.com', $topic, $fullmsg, $headers);

        if ($carbon == 1) mail($email, $topic, $fullmsg, $headers);
} else {
        wp_mail($recipient, $topic, $fullmsg, $headers);
        wp_mail('another@recipient.com', $topic, $fullmsg, $headers);
        wp_mail('and-another@recipient.com', $topic, $fullmsg, $headers);

        if ($carbon == 1) wp_mail($email, $topic, $fullmsg, $headers);
}

Alternately you could use Cc/Bcc by adding to the $headers variable, or even some combination of methods, whichever works best for you.