You are pulling agro because you are playing with terrbile tanks. They should be able to easily hold agro on the boss and a couple of adds. Also i had to unbind my agro dump ability since i never used it, i am not far in end game yet ( a couple hardmodes and 4/5 EV) but healing threat seems very small.
Also you have very good AOE heals as a sage, so tell your DPS to grab the adds (they should be doing this anyways) if they are on you so they can share tank heals.