telling mark/damage absorbtion

Maybe its silly but i have been wondering that.When in a group 1 target is absorbing the damage of the others,on who do you put your telling mark??

Lets take SG as an exemple...

1- you put it on one of the guard,everybody hit this one and brute get 10% more damage from this target,normal from the others

or is it

2- you put it on the brute since he's getting the incoming damage and all the damage done to the guards around hit him for 10% more

I have been trying it 2-3 times,not being able to tell whats working. Anyone ever done some testing and know it for sure??