Also, get used to each character and use them in a particular order. For example at the stage of the game you're in, throw Alastair into combat first (unless you're a tank yourself), then position yourself where you're most useful (mage - nearby, but not between alastair and more enemies; Rogue - in the thick of it, but move yourself behind the enemies so you're taking less damage and get the backstab bonus; Dog - with alastair, but try not to get aggro)
Aggro is "attention" of enemies. IE, a tank is best to get all the aggro, meaning all teh enemies are attacking that one guy, and everyone else can then attack the enemies without being hurt. If someone "steals aggro", then they've attracted too much attention (through high burst damage usually) and the enemy turns to attack that guy instead.
. There are conversation options at camp where you can ask party members to leave, though i've never tried one out.
