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.

noplanman

NOTE: This plugin serves as a mini-plugin until Tsu releases an official public API.
The Tsu plugin adds a widget to place a link to your Tsu profile in your sidebar. Your readers can then easily visit your Tsu profile and use it as your invitation to the network. 
You can optionally add a name to be displayed and choose if a logo should be shown in different sizes.
Translations: 
* English, German & Spanish (<a href="https://www.tsu.co/noplanman" rel="nofollow ugc">Armando Lüscher</a>) 
* Dutch (<a href="http://tsunl.nl" rel="nofollow ugc">Elza van Swieten</a>)

Tsu widget to share your profile / invitation with your readers.

Invite Friends to Register vs Tsu