How to restrict BU access based on the contact points for Bill management

Summary:

Hi team,

Could you please let us know how to achieve the following:

One customer account have 2 sites .. one for US and another for UK

US site with US RDS- Added user1@gmail.com as external user contact point. Invoices are raised under US business Unit.

UK site with UK RDS- Added user2@gmail.com as external user contact point. Invoices are raised under UK business Unit.

Invoices are available under both US and UK.

Registered both the email id's for bill management portal.

When I login using both the email's, I see invoices for both BU's. It doesn't restrict it from viewing invoices of other BU's.