We offer a repair service for production managers that look after several businesses each which needs their invoices billed out to their relative venue.
An example would be 'John Doe' is associated with 3-4 different businesses on our system and would like to link him (number & email) to those 3-4 business.
Maybe it could work at the point when adding a contact to a business it looks up a table of existing contacts on the database and autofills the details in.
From a 'Contact module' you could then edit/revise the links if the person has left the company or joined to another.