Tracking referrals - what's the best field to use?

Hi everyone,

We want to launch an email referral program and we want to be able to track the referrals to the original referrer. Basically, when someone forwards an email to refer one, two, three or more friends, we want to make sure that we are able to track back these referrals to that original person. One option we are thinking is appending a unique identifier/field (ie. contact ID) to the URL string, something that looks like this:


Is this possible? And if not, are there any alternative ways on how we can accomplish this goal?

Any idea/feedback is appreciated! Thanks!