Hi Phill, you can add an HTML instance to your home page, that's dedicated to inviting people to register. Put in some introductory text along with a register text or image link like this (updated per Joe D.'s message below):
<a href="/secure/register.aspx" title="Register" >Register for an Account</a>
If you want that register link to stand out, you can surround it with header tags like <h1>, <h2>, make it bold, use an image, etc.
After this, go into the settings of the HTML instance, and on the General tab, check the box that says "Hide from authenticated users." After that, if will only appear if you're logged out, so if you need to edit the HTML instance, you'll need to get to it via Administration, Content Manager.