Rotate Turret to Tag

Hey guys, I'm making a Multiplayer game with Photon networking. Well anyways, I'm not asking for advice on Networking, I can do that part.

What I'm trying to figure out is.

I know there is a Transform.LookAt(Name of Variable);

But that wont work the way I want, as I will have players leaving and joining games all the time.

I really don't want to put in an Update function to keep searching for new players when I know there has to be another way to do this.

I have a trigger zone, with an OnTriggerStay and OnTriggerExit.

Well, I have the tags of "Red" and "Green".

Well, how do I make this turret rotate to a tag that enters if I can't use transform.LookAt?

I need to have the model rotate towards player and have the BARREL rotate up and down just encase enemy is higher or lower than the barrel.

I don't have any code because I can't get anything I try to work at all. I know it's something simple, I just can't quite grasp how to rotate to only a tag that enters into the Trigger Zone.

I want an effect like this. As you can see, I have the top part that rotates left and right and the boxy looking part that lifts the barrel up and down.

I don't need an ultimate way of doing this, just a hint on what I need to look up to get this to work. As I've always used transform.LookAt. I don't know anything else to use.