Special Invitation Codes

Sorry if this is some place else. I haven't been able to find it yet. My question: is there a way to set up "invitation codes" that I could give friends to give out on their blogs/twitter/whatever, which would lead to special discount prices on my membership blog (and perhaps be trackable?). So I could give each friend a code (e.g. 0A32-2011 or whatever) that would have a discounted price when a person registered using that code--and would tell me where they came from?