Check out: http://www.highwayman.org/gmail ...
It's a free service, don't need to subscribe to any bulletin board. You need only provide an email address of where you want the invite code delivered. Each user will receive the 'actual' code needed to create an account
Secondary email address that Gmail asks you to set will be done automatically at the account creation stage.
The site also accepts donations from users with an invites to give away but don't want to be hassled by users constantly asking "give me an invite, please?".