A special type of external user is the "Sales user". A Sales user can be created by adding a "Sales Publisher account", to which you attach users, but no websites. These Sales users can then be edited and given access across the other Publisher accounts, so the Sales user can report on the Publishers they work with.
Once the Sales users has been created you can build up a Sales user portfolio, through Global Settings -> Users and portfolios
If you want to map one or several advertiser to a Sales user, you will be able to filter on how that particular sales user "perform" across his portfolio. The Sales user will also be able to filter on his mapped Advertiser, to see how well he is doing with his responsibilities; which can be very useful as reporting can be done on Revenue channels, like deals, OpenRTB or Direct campaigns broken down per website; as an example. The Sales user can also use the Advertiser reporting in combination with the Trend reporting, to look for Advertiser performance and potential new leads in the OpenRTB spend on different websites etc.