Affiliate Plus lets your registered users get credit for new users they recruit by sending people to any url on your wordpress site with ?affid=username appended to the end. This will set a cookie on the visitors computer, which tracks who they have been referred by.
You can select who gets the credit for the referral: the first affiliate who send the user to your site or the last (cookie is overridden with new affiliate ID) You can also specify for how long the cookie is valid.
Members can be redirected to any page you want when logging in (admin users will not be redirected)
You can see who has been referred by whom and how many new members have been referred by each user on the user list.