You can set up your Referral Program to be more like an Affiliate Program where users are required to apply before being approved.
To do so, you'll need to:
1) Set up the questions you want to ask in order to collect approvals by creating a Zero Party Data activity and setting it for manual approval
2) Design this into a Form component
3) Placing this component on a landing page
4) Approve the members - Now as you see members start applying, you'll need to approve them for them to be added to the segment.
5) Now to automatically add these users who are approved to a segment you will need to create a segment