Dennis

With this plugin registrations are only allowed if the user has an invitation code. This plugin is only for WordPress MU and is BuddyPress compatible.

Invitation Code Checker

With this plugin registrations are only allowed if the user has an invitation code. This plugin is only for WordPress MU and is BuddyPress compatible.

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.

Invitation Code Checker vs Tsu