Single Email for Multiple Customers


One of my customers is a Car Dealership, they use a single point of contact for all locations tied to the dealership. How might I be able to go about getting that contact the invoices and estimates for all 3 customers? Every time I attempt to add the contact to another customer it says you cannot do that. Is there no way for a contact to switch between different businesses in their portal? Or even just tag the invoice/estimate/tickets as such for the different businesses?