Restricted Registration w/ E-Mail Verification
$89 
Limit registrations to specific domains... Marketplace
Lockdown – Extends Force Login
$14 
Lockdown the site so no one but users you specify have access to any deals. Subscription Lightbox
$39 
Lightbox subscriptions... Registration Captcha (reCaptcha)
$17 
Simple way to add a captcha (reCaptcha) to your GBS site's registration page. Invitation Only
$59 
Restrict registrations to invited guests... Custom Registration Fields
$19 
This is an advanced plugin built for developers... Registration Rewards
$19 
Reward a new user by crediting their account automatically...