Great plug-in, works really well.
I just wanted to let you know about a little (not so important) error I have.
If a person want to register but something goes wrong, I see the error box showing up but it is actually empty.
Here is a screen to show you what I mean:
Besides that I wanted to give some feedback about the way you now add invitation codes. It would be cool if you could just add a bunch of codes in one go, instead of adding every single code separately.