Note: In this example the 3rd party will be called ACME Experts.

First, you would create a department for the users that work for ACME Experts. Create a user account for each ACME user and assign these users as employees in the ACME Experts department. If you want to have a person inside your organization review the referrals first and control the assignment of referrals to the third party referral recipients you can make an employee of your organization the manager of the ACME Experts department and gatekeeper the products assigned to ACME Experts. Otherwise make one of the ACME Experts employees the manager of the ACME Experts department.

Make sure that you configure the ACME Experts user accounts with these features:
Receive Referrals = Y
Group Account Feature = N
Search/Browse Referrals = N
View all Referred Products for Shared Referrals = N

With these settings in place you will prevent the ACME Experts users from searching through your customer database and viewing customer information.