Invitation only - Verification sign up procedure.

Hello,

 I am looking for a verification procedure which is of the utmost importance to my site and was wonder whether anyone could help me?

I need an invitation only policy on my website. This means that anyone without a username and password will only be able to get to a front page and not view anything else or progress and further unless signed in. (by front page I mean simply a login box with username and password fields)

What I would like is current members to be able to invite someone which will send them an email with a 'one time passcode' on it. Then when they click on the link they are taken to a sign up page which has the normal sign up details BUT also includes a space to write in the 'one time passcode' in.

The 'one time passcode' as it states should only be available only for that particular sign up and must expire if not used in 30 days.

Obviously should someone manage to get to the sign up page but not have a passcode then their sign up will be rejected.

Can anyone help me with this? Is there any product already out there? Does any one want to build it?

Thanks

Ronnie