Import contacts from the Address Book from Yahoo!, GMail, AOL, Hotmail and other providers. 
The plugin also provides all the nice features of OpenInviter:
<ul>
<li>auto-updates</li>
<li>both local and remote debugging</li>
</ul>
Supported services:
<ul>
<li>AOL</li>
<li>GMail</li>
<li>Windows Live (Hotmail)</li>
<li>Yahoo!</li>
<li>Rediff</li>
<li>Lycos</li>
<li>Mail.com</li>
<li>Mail.ru</li>
<li>Rambler.ru</li>
<li>Twitter</li>
<li>MySpace</li>
<li>Facebook</li>
<li>Hi5</li>
<li>Orkut</li>
<li>GMX.net</li>
<li></li>
</ul>
Services coming soon:
<ul>
<li>GMX.com</li>
<li>Email.it</li>
<li>Fastmail.fm</li>
<li>Canada.com</li>
<li>YouTube</li>
<li>(and many others…) 🙂</li>
</ul>

OpenInviter for WordPress

Allow your visitors to invite their contacts from Yahoo!, GMail, AOL, Hotmail and other providers to your blog.

Tammy Hart

This plugin adds a page to the admin under Users that includes a form for users to send email invitations to their friends to register at your site. You can also add the form to the front end of your site with the <code>invite_friends()</code> template tag or the <code>[invite_friends]</code> shortcode.
IMPORTANT: This plugin will not allow invited friends to register unless you have registration turned on.
<h4>Features</h4>
<ul>
<li>Invite multiple friends at one time</li>
<li>Customize the email subject and message that is sent</li>
<li>Customize the URL to redirect a user to after they register</li>
<li>Store successful registered users in the inviter’s user meta data</li>
<li>Store the inviter in the new user’s meta data</li>
</ul>

Invite Friends to Register

Use this plugin to allow users to invite their friends to register on your site.

OpenInviter for WordPress vs Invite Friends to Register