Hunters will also bring this buff in the form of Trueshot Aura, and warriors in the form of Battle Shout. Now if you have a hunter in your group, Trueshot Aura is passive, and bought by all specs, so you're done. You don't need to worry about keeping Horn of Winter up. A warrior's Battle Shout, however, is much like ours in that it needs to be recast often. If you have a warrior in your group, consider asking them to cast Commanding Shout instead, since you'll be covering that attack power buff with your horn of winter. Of course, Commanding Shout might just get overwritten by a priest or warlock, in which case it doesn't matter. Regardless, you should be using Horn of Winter regularly for that extra bit of runic power.
Unholy Aura was formerly Improved Icy Talons, and was bought by frost death knights. Now, unholy death knights can bring it too. It's passive, so we don't need to worry about refreshing it, but it's there, and it's nice.
Unholy Frenzy can be bought by unholy death knights, and provides a quick burst of haste to a targeted individual. A single target spell wouldn't normally be thought of as group utility, but in this case, it can be cast on others, so it counts. Before patch 5.0.4, it was also classified as an enrage, so it was most useful for fury warriors, who gain extra power when enraged. Now, it's not, so you can feel free to cast it on whoever could most benefit from the haste. In many cases, this may be you, but keep an open mind. Remember, it's melee and ranged haste only, so don't waste it on a caster.
Wearing them down with debuffs
As a DPS death knight, you can apply the Physical Vulnerability debuff to your opponents, allowing all physical attacks to hit a bit harder. It's applied by DPS specs only, by the spec's most thematically similar disease. For frost, that is Frost Fever, and for unholy, Blood Plague. Since there are very few situations in which you should not have diseases up, this is a very nice fire and forget it debuff, and one you'll always have up regardless of who else is bringing it.
Weakened Blows is the tank death knight debuff, and one which decreases physical damage dealt by 10%. This means it's a somewhat selfish debuff, since it directly affects your survivability, but that also means that DPS take less damage if they pull aggro, and the healer has to heal you a little less, so it's definitely worth putting up. Once again, it is applied by diseases, Blood Plague in specific, so there's very little reason not use it.
Both of these debuffs can be bought by a variety of classes, and even a few specific hunter pets. However, since diseases are such an integral part of our class, you should be putting these up on your own.
Slow casting is the more awkward debuff we bring, as it's connected to Necrotic Strike, which in itself can be very difficult for death knights to fit into their rotation thanks to the single death rune cost. Of course, PvP will likely want to use it anyway for the health absorption, but for PvE, the optimal happening is that you have a rogue who's using Mind Numbing Poison, a mage who's using Slow, or a Warlock who's using Curse of Enfeeblement. There's also a few hunter pets who can bring this debuff.
Even if you don't have any of those classes in a PvE group, you may want to focus on interrupting spells rather than slowing casting. It's definitely less disruptive to your own rotation than fitting in Necrotic Strike.
Talented moves and other wild cards
Control Undead isn't completely, exactly group utility, but it does exist. It can be useful simply as a spare crowd control method. If you control an undead mob, that's one less mob you need to deal with. Of course, since Control Undead only lasts 5 minutes, you'll need to kill it eventually, but you can always either have it suicide on its compatriots, or release it after everyone else is dead and your group is ready to deal with it. One downside to this ability is that it does replace your summoned ghoul (or geist). This isn't such a big deal for frost and blood death knights, but unholy death knights will at least temporarily lose the utility of Leap and Gnaw, and their DPS may change depending on the power of what they tamed. On the other hand, you do get access to whatever spells or abilities your tamed monster has, so the replacement may be superior, depending.
Anti-Magic Zone is a talent, so you do have to make a deliberate choice to bring it. In addition, since it sets up the zone at a specific point, your group has to be coordinated enough to run that point, and you have to set the zone at a place that won't mess up positioning in an salvageable way.
If you're facing a magic heavy boss or other magic-using mobs, if you don't think you'll miss the self-heal or fear breaking powers of Lichborne and if you can convince your group to run to the zone when you cast it, this talent can be just the thing to make you supremely useful to your group and make the difference between victory and defeat.
Desecrated Ground is essentially our own personal version of a Tremor Totem. If you're fighting a boss that fears or sleeps people, this might just be worth the investment. It's a level 90 talent though, so it'll be a deliberate choice to take it.
Gorefiend's Grasp, as a level 90 talent, seems mostly useful for tank death knights who want to wrangle in their prey. That said, it has two little twists going for it. First, it doesn't taunt. Second, it will pull the mobs to your target rather than you. This means that even a DPS death knight can target their tank, hit Gorefiend's Grasp, and send all the mobs in the area directly to him. Don't underestimate this as a nice little helpful tool to keep your dungeon runs a little smoother. Remorseless Winter could be seen to work on a similar principle. Keep the mobs slowed or stun them, and you or your tank can pick them up that much easier.
Learn the ropes of endgame play with WoW Insider's DK 101 guide. Make yourself invaluable to your raid group with Mind Freeze and other interrupts, gear up with pre-heroic DPS gear or pre-heroic tank gear, and plot your path to tier 11/valor point DPS gear.