As a Barbarian, how to be effective in coop?
There are a few Barbarian skills that charge at enemies and generate fury which should be used often. Leap is one you gain fairly early on, and you can even use it to jump up and down ledges or across chasms where other classes have to run around. You should generally be running ahead of players rather than lagging behind.
Also remember that you can build up Fury just by attacking things like crates. Keep your fury high even without any enemies around so that you're ready to go when you do find something to kill.
Barbarians (and Monks) get a passive 30% damage resistance to make up for their melee natures, and tend to have higher vitality and health than the other classes. Against many enemies, it is the right thing to do to just jump in and start causing havoc. Barbarians are good at distracting enemies while their teammates do damage, so try to focus on that rather than claiming the final blow. You get full experience and drops whether you're the one landing the finishing hit or not.
I have been designated as "The Barb" in a co-op group between me and my friends, and here is what I have learned from the experience so far:
The first thing to do is make sure your party members follow YOU into battle, not the other way around. You should be the first one in at all times. If they are running ahead of you that is their own fault.
Equip yourself with items that provide bonuses to life acquired from health globes, leeaching bonuses per kill, or leeching bonuses that give you a % of health per damage done. Couple this with your usual stat preferences of strength and vitality, and you should have the confidence to charge into the thickest part of the battle.
Leap is your friend, however, use it wisely. Sometimes it is better just to run into battle first, let the baddies surround you, then leap right in the center. This will stun all of them. Follow that up with a Rend and start cleaving your way through them.
Your warrior is most effective when surrounded. It will provide a limitless source of fury, and will often trigger your revenge skill (if equipped) that causes massive damage.
Defend your friends, they are counting on you. If you see a powerful mob making a bee-line for a friend, use your grappling hook to pull them back to you. Alternatively, if you don't have a clean sight line, use your Leap ability to get over there. Ranged characters are relying on you to dictate where the center of battle is. They have the mobility to adjust to you more so than you can adjust to them.
Here is my preferred DPS cycle for large groups of mobs:
Leap - Rend - (Revenge any time it triggers) - Cleave until Rend needs refreshing
Keep in mind, as the game gets more difficult, you will be expected by your co-op group to be a better tank than a better damage dealer. This will mean switching to a 1h and shield combo. You will need that extra armor to survive in the thick of all those mobs.
But once again... the golden rule of co-op for barbs...
You are TANK first, everything else is secondary.