There's a free module out there that let you do the same thing. Unfortunately, I can't remember the site's address. May be you should check the Announce It! forum.

However, you can also set up a signup page manually and this is really easy. The simplest way is to enable the "Anonymous signup" in Host settings, then provide a link to the "Add new portal" page in the Portals tab. The signup template can be customized in Signup.ascx in the folder admin/Portal.

Moreover, it is possible to create an independent Portal-signup module so you can use it wherever you want. First, copy all of the files in the folder admin/Portal to a folder inside Desktop Modules (for example Portal Signup). Then go to Module Definition tab, select Create New Module and name the new module whatever you like (Portal Signup). Next add a new definition to the module : name it Signup, specify the source value to DesktopModules/Portal Signup/Signup.ascx, type to View, hit update and you're done! To use it, just add it to a page like any other modules.

Hope this helps!