<?php

if (class_exists('wpMailinglist')) {
    if ($wpMailinglist = new wpMailinglist()) {
        global $Subscriber;

        $subscriber = array(
            'email'             =>  "email@domain.com",     //the email of the subscriber
            'list_type'         =>  "specific",             //always leave this "specific"
            'list_id'           =>  "1",                    //the ID of the mailing list to subscribe to
        );

        if ($Subscriber -> optin($subscriber, true)) {
            //subscriber was saved!
        }
    }
}

?>