Beta Invitation Codes

[eluser]Vheissu[/eluser]
The way I have done it in previous projects is:

A beta codes table:

id - auto increment, int 10
beta_code - A random 16 character string md5 encrypted
expiry_date - An expiry date for the code
email_address - varchar (120) optional column in your table for restricting code to an email address
code_used - int(1) 0 for no and 1 for yes, to keep track of codes used

User visits URL: http://yourdomain.com/register/98sdhasjk...4jhkjhfdsf

This would hit a register controller expecting a code in the URL:

The code would be checked against the table, if it exists allow registration and if not, show an error message. I might release my beta codes library I wrote a little while ago for Codeigniter when I get the time to make some tweaks for others to use it. Good luck.