You need to click on the affiliates tab at the bottom of any hubpage when you're signed in and follow the intructions for creating your unique tracker.
Once you've done that go to the sign up page: http://hubpages.com/tour/hubpages/go
Look down at the bottom of the page and look for the "link to this page" tab. Click it. A new box will come up with the sign up page's url with your tracker included.
Then send you friend an email including the link you have just created. Once she signs up by clicking on the link you gave her you will be able to see her in your referrals by clicking on "URL trackers" in your account.