Warriors - Hold the LineFollow

Warrior specific discussion here, as I don't recall seeing crusaders or monk classes getting hold the line.

This tool has been simply great when it comes to holding aggro on a group of mobs. My question, and possibly as much my observations, is.. does this build aggro on more than one group of mobs.

Let me further explain. In comes one group of 3 mobs, that are all part of one pull.. i.e. they all highlighted when I targeted one of them. Now, I am certain hold the line aggro's all of these mobs when I do damage, I have even noticed them all get red in the face when I do so. Then, lets say an add comes (can be one double up arrow mob, several mobs all in one group). These mob(s) are obviously not part of the initial pull, so does hold the line affect these mobs?

From my observations, I would say it does not, unless I target and damage them as well. This means I now have two levels of aggro (one for the first group, and one for the adds). Since the combat in this situation is usually faced paced, and I'm struggling to try and get all the mobs aggro'd, I haven't been able to notice if mobs from both groups (initially pulled one plus the adds) turn red in the face when I hit any of them. Can anyone confirm HoL does or does not affect a group of adds (or more specifically affect a mob or group of mobs which I am not attacking, since I have two groups in camp)?

Yellow icons effect yourself
Red icons effect another party member, or an NPC
Green icons effect a single ecounter, solo or group
Blue icons effect your party, or all NPC's in range