Coupon Codes - Unrelated to SEO but looking for functionality

As title says: Question is unrelated to SEO - but:

I am looking for a coupon-code system used by vBSEO for my board. Apparently hard to come by so at the off-chance someone can point me in the right direction?

Coupons to be provided to entice new 'valued' members to sign up by invitation, without having to pay for membership fee(s).

I'd be over the moon if someone could provide suggestions and/or advice on this.

Thanks in advance!

Our system is a private one, so we cannot help you there. I have not seen to many around for vbulletin.

Sorry.