Try messing around with this:

var vel: Point = new Point(target.x - pos.x, target.y - pos.y);

var velLength = vel.length();

vel.Normalize(characterSpeed);

if(velLength < 10){

vel.x = 0;

vel.y = 0;

}

and when updating the character increment it's position by the vel aka velocity.

Do that for each of the units. You'll notice this is just vector math.

Also if you want to study it. Just look at Boids. It's a group of algorithms for flocking behavior. If you study it long enough it should make sense. You'll end up with something like:

http://www.assaultwars.com/flash/AS3Boids/AS3Boids.html
It's a simple set of algorithms but it takes a lot of understanding to use them right.

//

here
//also since people doing AI ask about steering it's crucial you understand the

dot product of two vectors.
and here's the algorithm basically once you understand dot product.