Currently there is no data linking between Employees and Customers/Sales Invoices, however if you re-read @tut’s response there were some work around options provided.
In Customers you could include the employee’s name in the address panel after a couple of line spacing’s - then their name will appear on all Sales Invoices for that Customer.
In Customers you could create a custom field called Sale Representative where the Employee’s name could be entered, however for this to flow through to the Sales Invoice you would need to build a Sales Invoice custom view template.
On the Sales Invoice you could create a custom field called Sales Representative, this would appear as a blank panel at the bottom of the Sales Invoice where the Employee’s name could be entered.
PS: There is no need to put down a response, perhaps your post isn’t that easy to understand,.