If you use unit frames like PitBull and you use [target=focustarget] for all your macros, you can have one of your party target the extra person. In PitBull have the Party Target module enabled. You will see that party member's target all the time unless they un-target them. I think there is even a module for Party Target's Target.

I use this in world PvP when someone is trying to help me. Its nice to see if they need a heal or whatever.