DKs have some excellent healing skills; one burst emergency heal that scales with Max Health called Green Dragon Blood and one heal over time called Cinder Storm. They also have a group shield called Igneous Shield which scales with Max Health, this also restores Stamina while blocking via the Help Hands Passive and gives the caster Major Mending which boosts the healing they can do.
The DK has access to 3 unique group damage buffs called Stagger and Engulfing Flames. Stagger places a debuff on enemies via the Stone Giant skill which causes the target to take increased damage. Engulfing Flames can make an enemy take up to 10% more Flame Damage from your group. DK’s also have a Stamina group buff called Minor Brutality which is often applied by casting Igneous Shield. Stagger is more common in the later stages of the end game due to its difficulty to maintain and sustain but is a requirement by most groups in harder content and trials.
DK’s can also offer groups Major Brutality and Major Sorcery with Igneous Weapons, which is a great skill for all groups since players in lower tier groups are unlikely to be consistently using strong potions, and in higher tier groups it gives them the option to run alternative potions such as Ultimate gaining potions to boost damage even further. The DK also has another unique buff with Engulfing Flames which increases the amount of fire damage an enemy takes. This buff is better maintained by a DK Magicka Damage Dealer, but of you are the only DK in a Magicka Group you would likely need to use it.
A few things to remember is that if you are trying to optimise a group, the DK Tank should only be considered if the person playing the DK Tank is maintaining Stagger with Stone Giant if no other DK is doing it, maintaining Engulfing Flames if they are the only DK in a Magicka group, providing Minor Brutality in a Stamina group or if the person using the DK is doing so because they need help managing their sustain. If none of these things are applicable then you should consider a more beneficial class for optimising your group.
Overall the DK is a very strong class to play as a Tank. If you are new to Tanking then a DK would be the best option by far, purely for the sustain benefits. They are top tier for 4-person content due to their CC abilities and they have a lot to offer in group content.