This is an example of adding a very simple invitation code to your registration process. In this example, we will add an invitation code field to the form and set up a function to validate that code. To keep it simple, the invitation code will be static (there will be only one code). Later I will be publishing a more advanced example where you can read user specific codes from a database table, validate, and also mark the code as claimed.
This article pre-dates the Invite Codes plugin extension. I am leaving it up because it is a nice, light-weight method of setting up a simple invite code process. However, if you just want something easy, check out the Invite Codes extension.