As a chanter who also thought the mark NPC ability is a godsend, heres a few pointers.
You have to be the group leader to use the ability, but if you also get the 'delegate main assist' ability, you can allow for the tank to do the marking instead. Also if you point out you have the ability, people will frequently allow you to be group leader if you just ask, most people dont really care.
And finally, tho I am not certain, I believe people that do not have GoD dont see your new indicator, so sometimes you will still get group members that dont see your targetting and wake up the wrong mob, but its a definately improvement.
-Syresia of Nameless