There are 2 types of amulets to reset your points.
1) you will notice only a certain character can use.
2) you can buy generic ones from the Blacksmith, as you mention.
- buy them from the blacksmith, hit "I", go to accessories, just like you do for your main character. Simply use the arrows under the character's name to swap to a different character and "equip" the necklace to that person's slot. It will get used and their points will be reset.
So the same way you do your main guy, just change toons in the inventory screen.
I played DA1 as a tank with 3 ranged AI following me around blasting stuff.
Now I play as Verric and have 2 tanks to run around and grab/hold aggro while myself and the range mage just pummel from afar.