add_action( 'user_register', 'atuser_info');
function atuser_info( $user_id ) {
    global $wpdb;
    //$prefix=$wpdb->prefix;
    $query_userinfo = "INSERT INTO user_info (id,fname,lname,address_1) VALUES      (".$user_id.",".$_POST['fname'].",".$_POST['lname'].",".$_POST['address_1_step2'].")";
    $wpdb->query($query_userinfo);
}

$user_id = wp_create_user();

if( is_wp_error($user_id) ) {
    echo $user_id->get_error_message();
} else {
    //add into custom table
    echo $user_id;
}