What about a sticky? something like "Enjoy Racing? Have friends who might? Why not invite them to join RD?"
You could then say why not copy and paste this into an email to you friend? This could include clickable links to RD, but given many peoples understandable reluctance to click links within unsolicited email, RD's easily memorable RaceDepartment.com addy I don't even think thats necessary.
Perhaps then making the NEW VISITOR link in the front page more prominent with a drop down tailored specifically to lead new visitors to the site to areas of particular interest to them? Maybe by selecting a tick list that then presents them with a list of relevant threads.
I don't know if RD members would be any more tempted to refer people to RD in this way, but I don't see how such a sticky and helping new visitors to the site could be a negative thing in anyway for RD. Plus having a specific sticky and a well written body of text telling people about RD that members can just copy and paste into an email makes it much quicker and less time consuming and hence more likely to happen...
Why not just adding a button in the menu under "Community" and a simple form with a textbox in which to write the emails of your friends separated by ", " like Lockerz has. It should be pretty simple to get done. If you want to reward people add to registration.php?ref=user and just modify the registration script. I've done this for phpBB3, but dunno if it is allowed by vB's licence.