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.

Tyrel Kelsey

Automatically invite users that have filled out a specified form to your Slack team instead of having to manually invite them yourself.

Formidable to Slack

Allows site owners to automatically send Slack invites to anyone that fills out a specified form.

OpenInviter for WordPress vs Formidable to Slack