You can select multiple targets for any one action. This can represent things shooting several people in quick succession or intimidating a group of people by screaming at all of them.
In general, you first specify how many people you try to affect and take an ActionValue penalty equal to the number of targets you choose. You roll the dice and apply this effect against the first target, then reduce the ActionResult by one and apply this number to the second target. Continue like this until you run out of targets - if the difficulty is different for different people, you may fail against some early targets and succeed against later ones with weaker defenses.
Mook Interactions
You do not generally suffer multi-target penalties when interacting against groups of mooks. As long as the GameMaster judges your stunt and your targets reasonable, a group of mooks generally counts as just one person for interaction purposes. On the other hand, Mooks are generally less affected by interaction than other characters. See Stunts.
Area Effect
Some attacks and effects inherently affect an area - they always attack all targets in the area, usually not discriminating between friends and foes. Such attacks do not get a ActionValue penalty for attacking several targets. Instead, you attack the first target at your full action value, and each succeeding target is attacked at Action Value -2, comulatively. Third target at -4, fourth at -6 and so on.
Unlike a normal multi-target action, your targets get to decide which of them is most vulnerable and affected first. Also, if somone decides to do a "defend other" action, the defeisive value thus generated applies to the whole area effect - this assumes a stunt like "covering fire" or "jumping on the grenade".
If targets are very densely packed, an attack bonus might be appropriate; +3 per 10 targets is about right.
If friends or bystanders are in the area, make a separate attack roll that applies only to them, as if you made two separate attacks - one against friends and bystanders, one against enemies.
Autofire
Autofire is a special case of area attack that allows you to hit several times against the same target or different targets in the same general area. You must fire three rounds with each attack, and may not reload during the attack sequence.
If firing on several targets, use the normal area effect rules.
When attacking one target several times, resolve each attack separately, but there is a -2 penalty on each successive attack. For each attack beyond the first, you must expend one additional shot hosing the target down, but you perform all these attacks simultaneously. You also get one "free" attack against each friend or bystander in your line of fire, with no penalty.
Once a target starts to use ActiveDefense, he gets to apply the Dodge bonus against all your attacks, as they all come in the same shot.