Spark Feature Request: "Invite a friend'

This is something I think would benefit the majority of Spark developers, leverage your existing users to help grow your user base. If your users love your site, they will probably do this anyway, especially if the site benefits from having a large user base, but why not make it super easy and provide additional incentives out of the box?

Route to send invitation e-mail. Easy way to give the user a credit if the friend they invited signs up. This would need to have the option where the incentive is different depending on the type of account the friend signed for. As a simple example, say you have a free plan and a paid plan, if the friend signs up for the paid plan, the person they invited gets a discount on that plan, or maybe a free month.

Not sure how to do the actual implementation. I was thinking the person who was invited has a field to plug in the e-mail of their referre, but maybe the invitation e-mail includes a link back to the sight that includes the referrers user ID, and when that person sets up their account that ID is automatically saved. Then, if they create their paid account the person who invited them can get the credit.

Anyway, seems like a great basic feature which fits really well into Spark's feature set.