Registration by invitation only. (coupon code) Is it possible?

I’m currently working on developing a website where I would like to allow membership registrations but by invitation only.

Basically the way I want to handle this is by way of a pass key.

When the user registers, the pass-code is checked against a list of active keys. Each key can only be used once, so once that key is used up, that key is deactivated and no one else can use it to register.

Is there anything similar to this available as a plug in? or is there any help or suggestion anyone can offer me so as to implement this feature? Any help is greatly appreciated, thanks.