I have the same problem, I'm playing a melee rogue so I'd like to get my heavier warriors attacking before I join the fray and they just sit there when I tell them to attack (target enemy -> hit R or select skill). They will perform a swing or attack but they will do it in current positions instead of entering range of enemy and performing attack. It is annoying when they drop a heavy hitter opening skill 20 meters from the nearest enemy.
The only way for me to force an attack is select a ranged character and attack. Kinda of annoying as I'd like to mobs to not nuke the mage until CC has been achieved. The AI settings are quite useless as they are too one dimensional. I've tried defend...follow.... and they are all useless... I mean, wtf is that?! . I don't want the herd following my rogue, I'm supposed to be behind the scenes. Only way to get any kind of order is play in tac mode, which I feel kills the ambiance and flow of the game. ( I'm not playing on nightmare). I don't need a tank + side tank+ mage to help my rogue kill a archer when there is a tougher mob wacking the living daylights out of my party.