There are many buffs that give benefits to your allies, and debuffs that take remove benefits or add hindrances to your opponents. Read all of the passives of all of your skill lines, and ensure that you are keeping your needed buffs up at all times.
A simple example that is not used near enough is Choking Talons. Most DKs use the morph that adds more damage, though the Choking morph reduces opponent damage by 15%. If your group is going to charge into an opposing group of 20+ players, having a tanky DK go in a few seconds early and cast Choking Talons a few times before you hit them can make a huge difference.
The timing on your own buffs is also very important. If you’re using DW (either as stamina or magicka) then the cast of Quick Cloak just before you’re going to get hit by AoE damage will negate an additional 20% of said incoming damage. When I lead groups, I’ll often call for “20 second buffs”, meaning that I’m giving people 3-4 seconds to apply everything that they want before going in. I’ll sometimes call “20 second buffs” as we disengage or if the engagement is delayed by a few seconds to ensure that my team’s buffs are all up for as long as possible without them needing to spend casting time reapplying them.
There are also some sets that provide buffs to your allies such as Spell Power Cure (increase spell damage). Having such a set on a player who can provide the associated buff to as many players in your group as possible can greatly increase the performance of your group.