Would it be possible for one new customer to be referred by more than one existing customer?
You'll need a self join relationship here, but it may also need a join table if more than one existing customer can be credited with referring the same new customer.
If one an only one person can refer a new customer, this self join should do the trick:
Customers::CustomerID = CustomerReferrals::ReferralID
In Manage | Database | relationships, make a new table occurrence of Customers by clicking it and then clicking the duplicate button (2 green plus signs). You can double click the new occurrence box to get a dialog to appear where you can rename the new occurrence box as CustomerReferrals like I have.
We have not duplicated a table. Instead, this is a new reference to the same table already present in your database.
Now a portal to CustomerReferrals will list all customers referred by the current customer record.