Of note (useful for healers) if you target the main assist, and use auto attack, you'll automatically attack that guy's target. ditto to offensive spells, but you'll still be targetting that character for beneficial spells etc. (aka 'Implied Targetting')
just taget who ever u want to assist and all attacks made will go to whoever they are targeting .. this is great for assisting the main tank healers can heal at will and throw in a nuke or others spells on mobs also without having to switch targets