You could group them by employee in the portal by sorting the portal records, but an added "sub head" with the employee name is not possible inside the portal unless you also added "dummy" records in the portal's table that served as the sub heads. I'm not a big fan of that approach myself.
If this is for an invoice that will be printed and presented to the customer either in hard copy or as a PDF, I'd not use a portal for that layout at all. I'd use a list view layout based on the portal's table for printing/PDFing the invoice. In addition to many other benefits in using this layout, you can then sort your records by employee and use a Sub Summary part to display the employee's name as a sub heading in the invoice.