Invite all feature is best option for viral marketing of your website/ product. If you site is having invite all function, users will invite their friends and bring many new users for free. When you try to get this many users from any PPC marketing you will need to spend in sum thousands dollars.
Unfortunately, address book holding companies (Yahoo, Gmail, MSN) do not provide direct import features/ source codes. All they give is API. Not all web masters are coders. So here I am releasing easiest embed type Yahoo address book import source code.
The source code is written using oAuth in PHP. One of the main feature of the script is no password is required.
Step 1: Create Application key for your domain. Select Contact API when you are asked which yahoo APIs your app needs.
Step 2: Download source code of Address book import http://www.terabug.com/files/yahoo_invite_all.zip
Step 3: Edit config.php and replaced required parameters, keys that you got in step 1.
Your Yahoo Invite All Friends tool is working now, just embed it anywhere you wish to.