Ask HN: How do you manage beta invites for your web app?

I am almost done with the first version of my web app. I am planning to test my app with various 2-3 very distinct user profiles within my network. I would like to use some kind of custom invitation code system to track usage by different user groups. Any suggestion on good invitation code app which will let me create, share, & track custom invite codes/URLs? I have looked at prefinery.com but am looking for more options. Thanks!