Custom Validation – Entry Code

I want new users to be forced to register with a predefined ‘invitation’ code – we need several different codes available so we know who invited them. I think it is possible with custom validation, but I do not know how to make it work, I have tried a few examples from the documentation but to no avail.

Just 6 code would do to start with and without a valid code entered in the field, it would simply say “Sorry, your code is invalid”.

Codes: (I will change these) apple | banana | carrot | pear | potato | orange

Can anyone help. I need to know what code to use, where to put it and how to set up the custom validation field please.