The most common role for Venomancer's in any squad is to either act as a tank (If you have a Herc or a higher-than-the-mob Mag) or to pull mobs out from a group. Since the former requires little explanation (Just Spam heal and hold aggro) we'll focus on the later.
First thing you want to do is get yourself a fast pet, that is a pet that is 8 m/per second or higher. You want something that is light on it's feet and able to get in fast, taking as little hits as possible before it can 'tag' the mob you want to pull out. Examples of fast pets include...
*Note that most flying pets can't be called in dungeons, so while it is always good to have a air/sea pet, they aren't all that practical.
Personally, I use the Kowlin as my pulling pet, since it has a bit more stamina than other pets and a decent Mag Defense. It also has a speed of 10.2 m/per second.
Once you have your pulling pet ready, let's take it to the field.
First step is to take out your pet and target the mob you want to pull. make sure that you do NOT double click it and start running towards it. That is a very bad idea. You just want to select it out of the crowd at this point.
Next is to click the "Attack" button on your pet's hotkey menu.
Now comes the tricky part. You need to carefully watch the enemy mobs health bar. The very second you see it start to drop from a single attack, you know that the mob has been 'tagged' that is even if your pet is recalled, it will still want to fight.
At this point, recall your pet back. By recall, I mean click on the pet's icon in your hotkey menu so that it vanishes from the field. Just remember to recall you pulling/lure pet ASAP, if it's out too long, the other mobs may put their aggro on you, and recalling it will not get rid of this aggro!
The rest of the mobs will act as if there is no attacking force and go back to what they were doing before. But the mob that you were targeting will come running out of the pack toward you!
At this point, you have a couple of options. If you are squading with people, you can run back a ways and let your tanks/damage dealers run to the approaching mob. If you are soloing, you can pull a pet from your bag and engage the mob when it's out of the group. The choice is yours!
And voila! You've just completed a successful pull
Use this technique to pull mobs away from a boss or a boss away from mobs. You'll be on your way to being a very useful veno.