It is my understanding that the NPCs have a type of 'hit-list' if you will. Even if a player has 17 NPCs after him/her (those 17 NPCs have that player at the top of their hit-list) the other agroed NPCs, who now have no one to attack, don't suddenly put an invisible player (someone who has not generated any threat/attention) on their hit-list.
Anecdote:
I often farm in a duo where one character is an invisible buffer. Even when my partner gets more-than-cap agro, I never get attacked.
Computer has it right I've pled enough toons for pvp to know that if you go over the aggro cap NPCs don't magically start attacking someone else on the team just for being there. And I don't invis my alts that just sit there and they still don't have NPCs chasing them.
Anecdote:
I often farm in a duo where one character is an invisible buffer. Even when my partner gets more-than-cap agro, I never get attacked.