Working well as a team is incredibly important in any team game. It stands to reason that if you're not working as a team you're just a collection of individuals who aren't opposing one another. You're not going to win unless the other "team" is worse than you or each of your players is better than their opposite number.
In this short article I'd like to share some tips about working effectively as a team on games like Halo 3.
My first tip is about communication. How many games have you been in where everybody on your team has been shouting? Everybody shouting\talking at the same time can take your attention away and leave you open to sloppy shooting and getting flanked. The counter this, I've found, it's best to only have 1, at most 2, people making the calls. Which brings me onto my second point. Calling. I bet you've been flanked before and taken out because somebody died and didn't tell you that the player was coming. Calling is the act of telling the rest of your team about the whereabouts of incoming enemies and the weapons they're carrying. That person can also help to direct the teams fire on to individuals, to bring them down more quickly, by calling out the name of an attacker (or his position) and telling everybody to pour fire in that direction. Another thing I find useful to have going on is anybody on the team calling out if a weapon\grenade\power-up has respawned for the player best with the weapon to pick it up.
My second tip is about weaponry and what your team is carrying. The ideal weapons for your team to carry will change from map to map; but on most maps the general plan should be effective. Most combat happens at a mid-range. Not sniper range. Not shotgun\mauler range. Because of this I like to have 2 of my team carrying battle rifles for mid-range work or assault rifles for mid-to-close range work. The other 2 people on the team should be carrying short and long distance weapons. For instance, one person might take the sniper rifle\beam rifle and another would take the shotgun. The reason for this is to attempt to remove any weak ranges in your armor. Teams getting in too close won't be too much bother for your rifles and shotguns; but teams staying at a further range won't stop your rifles and snipers getting kills.
My third, and final, tip is about positioning. Staying in a close-knit-group can have devastating effects against a team with accurate grenade throwers; but spreading out can mean you're too far away to support each other. For this reason it's a good idea to work out far you can go from each other before you're not close enough to be blown up all at once but not too far give a helping hand.
I hope my 3 tips - gleaned from experience - have helped you on your way to domination.