As far as I've seen, when I'm pulling, if I am alone in the party, or if my party member is not near me, the monster I pull won't call for help, thus I will pull only 1 monster.

If I have duo party, and my buddy is near me, when I pull, usually the monster calls for help from 1 of ANYTHING that's around it. Say, I pull a Yarzon, and it called a Coblyn =_=;; However, if what I heard is correct, the devs team is looking into this and MAY make it so only same species will link.

Anyway, when I'm in 4 people party, if I aggro or pull one monster, it will call like 3 friends into help (if there are other mobs around it), because it thinks it's unfair that we 4 were going to band together against it. Yeah it's 4:1! So monster will try to make the force even.

So basically that's how it works.