If you've ever just joined a random CTF match in Halo PC, you know how tough it is to communicate with your team. Why? When you try to talk to your allies, you have to use the keyboard, which prevents you from being able to do anything (e.g., go after the enemy who's running off with your flag). In other words, you can't 'walk and talk.'
Most players respond by acting individually, and they're not necessarily making the wrong choice. You have to use discretion when deciding whether to give your teammates a vital piece of information or to act on that piece of information by yourself. I believe, however, that players too often choose the latter.
Beyond being more quick to communicate with other players, there is little else you can do to promote teamwork in CTF (especially when you've never before seen the six people you're working with and will probably never see them again). Here are a few guidelines to follow if you want your teammates to work together:
1. Never leave the base in an empty warthog.
2. Never leave the base as soon as you respawn. Wait for one or two others to respawn and head for the enemy base together.
3. If you arrive at the enemy base alone, wait for teammates to arrive. (They should do this without any input from you. Depending on the situation, use them as cannon fodder or act as cannon fodder for them.)
Regardless of your strategies, most attempts at teamwork are disappointing. There are usually too many noobs on your team, or too many people who just want to snipe the noobs on the enemy's team (without providing any strategic advantage). But I digress...
Because of the failure of teamwork, personal initiative must triumph. You have to go to the enemy base, cap the flag, and bring it home with little or no help from your teammates.
To do this you'll take advantage of the fact that the communications blackout works both ways--the enemy cannot coordinate its efforts to keep you out of the base or to kill you after you get the flag.
The first rule of capping is stealth (get active camo!). On Xbox, this meant sneaking into the enemy base without being seen, i.e., don't unnecessarily kill people on your way in. On PC, this means the same thing, but now you have more lenience in killing people, since if you kill someone, only that person will know that you're about to enter the base (he probably won't tell his teammates, he'll just come after you as soon as he respawns; keep in mind that you have to kill him inconspicuously: flamethrower, pistol, sniper, or frag grenade).
Stealth also means acting unexpectedly. For example, in BG, don't come straight up the middle. Go through the caves, skirt the shadowy area, or crouch walk along the ridge. This goes for vehicles as well. People don't think about the fact that vehicles can go inside caves or up on that ledge, and they'll be bewildered as to how an enemy vehicle got behind their base without their knowledge.
Continue to act unexpectedly once you have the flag. People go the same way every time they grab the flag. How many times on Hang Em High have you seen someone steal the blue flag and then jump down until they get to the active camo? Everyone does it. Likewise people follow almost preset routes on other maps if they have a flag. The solution? Don't use the preset routes. Instead:
1. Double back.
2. Double back.
3. Go out of your way (you're enemies will think you'll take the most efficient route to your base.)
4. Hide (if waypoints are enabled, this can sometimes backfire).
5. Double back.
What if someone who stole the blue flag on Hang 'em High jumped on the lower cat walk instead of going all the way to the ground and then went through the corridor under blue base (the one that leads to the shotguns)? It would confuse his pursuers and increase his chances of scoring. Believe me, it works every time. Here's another example--steal the red flag on BG and go through the telly, but go up into the OS cave instead; wait a while, and when it seems safe make a break for it.
Okay, the second rule of capping is strength. No one will expect you to be very aggressive as a lone person against an entire baseful of enemies, so it sort of ties in with the first rule. Strength means go in there with an OS and a heavy weapon or a sniper if you prefer, but do as little fighting as possible until you get the flag. Once you get the flag, don't simply try to run if you have enemies on your tail. You can't count on your teammates to get them off your back. Unless you can outrun them (in a vehicle) or if they are too far away to really hurt you or if you are close enough to scoring that you can make it before you die, (deep breath), then drop the flag, fight 'em, and pick it back up. You don't have to kill them, just throwing a grenade will buy you some time. I'd recommend killing them, as long as there are only two, though. They won't expect a fight, and you probably have better weapons than they. In close quarters, just melee with the flag, but in the open drop the flag and let them have it. Dropping the flag distracts them, and they might try to return the flag instead of concentrating on killing you.
Okay, the final rule of capping is speed. If you get the chance to jump in a warthog or banshee and race the flag back to your base at top speed, by all means take it. In fact, after using your stealth to enter a base, you might want to hang around until there's a nearby vehicle that you can jump into. A note on using banshees, though. If you have the flag in a banshee and there is a tank shooting at you, go through the wildest evasive action you can muster. You cannot outrun a tank shot.
Okay, one last tip before I go. A key to online CTF strategy is defense. Because most of the action in the game is based on personal initiative, there will be very few people noble enough to hang around base and defend. The ones that do are usually noobs, so your team's only real defense is a lucky death at the enemy base and a subsequent respawn in time to stop the enemy cap. Therefore it is up to you to sacrifice for the team and guard the flag. If your enemies score two quick points or a point in the first few seconds of the game, you can be sure that only noobs are guarding your base and that you have to stop your capping attempts for a while. Grab a good weapon, find a hiding spot in or outside your base, or just run around inside the base so you won't be caught unawares. Defend the flag for about ten minutes. This should be discouraging enough so that the opposing team will slow its offensive enough for you to go out and try a cap or two.