How to use the Rules module to award userpoints to referring users?
Add a field to user registration form that collects the uid of the referring user. Then ask current users to invite other people to the site using a custom URL to user registration form and this URL should include as query parameter the uid of the referring user. Hook the user registration form and fill in the uid of the referrer based on the query parameter. You should hide the referrer field completely. Then you do counts in the database for how many paid users a given uid had invited. The referrer uid in regard to each user is kept in the custom user profile field and becomes meaningful in your situation if the user becomes a paid member.