Super Refer A Friend is plugin which adds a new widget to your blog which allows readers to email the page they are on to a friend via a popup box. They can enter more than one email address and customise the message that is sent. The plugin includes a captcha and you can set a mailing limit to prevent spam
Support at http://wplift.com/refer-a-friend-wordpress-plugin
このプラグインをダウンロードして、インストール型 WordPress のインストールに使用できます。