This is a guide to prepare for veteran group dungeons for new players.

General information:
*Don't be a vampire. Just because you can, it does not mean it is a good idea. Remember that this is group play.
*Be in your best gear.
*Bring a minimum 30 soul gems with you.
*Bring 2-3 buff food with you, the dungeon might drag out to several hours.
*Bring a minimum 50 potions with you.
*In group dungeons, mobs and bosses are a lot harder to beat, than in normal dungeons and outside world. Some bosses have special mechanics, that require the team to work together. For instance:
Fungal Grotto has a boss, that will spawn a portal that takes in the closest group member in to a room full of spiders, that group member can not leave untill all the spiders are dead. - In this fight, it is usually the dps that has the best survivability that walks in, as he will not be able to recieve heals from the healer while he is in there. It would also be silly to throw the tank or the healer in, because then who will tank the boss or heal?
*Tell your group members, if you don't know the strategy on a specific bossfight.
*Try not to stand in stupid. There are some red circles where no amount of healing will save you, if you continue to stand in it for over 3 seconds.
*Don't rush into fights and wait for your team to catch up or regain resources.
*Make clear what your role is, before you join a group - so you know what they expect from you. If you are a dps and yell TEMPLAR LFG, then you will most likely be picked up by someone thinking you are a healer, and when they see you don't heal they will kick you. Yell instead DPS LFG.
*Don't aggro before the team is with you, if you rush from the boss into a mob and walk further, you split up the team and die, and cause your team mates to die as well.
*If a dungeon takes too long and you wipe a lot, let 1 member stay inside and the rest of you port out and repair, once you get back then the remaining member can port out and repair. Because if your armor is too damaged, the glyphs on your armor stop working and it will not provide you any protection either - that means a lot less damage and survivability. Dungeon completion can be questioned, if you are down to 0 armor.
*Sometimes dungeon completion is a goal in itself, if your team is really struggling then don't expect to succeed the dungeon with goldkey challenge on top.

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There are 3 main roles:
-Tank
-Heal
-Dps/DD (damage dealer)
Dungeons can be done with mixed groups, however the classical setup in ESO is 1 Tank, 1 Healer, 2 Dps.

If you play as tank:
*Have a backup self heal
*Slot at least 1 taunt skill, so you can aggro bosses and mobs.
*Focus holding aggro on bosses, as the healer and dps are squishy and can be one shotted.
*Make the bosses face AWAY from the Healer and the Dps, if given the chance that you can stand still with a boss.
*Try to do some form of damage or crowd control, while you fight adds.

If you play as healer:
*Your main job is keeping everyone alive. You can do backup dps, but do not forget that your team is depending on you as healer mainly.
*Have healing skills slotted on both your bars.
*Keep HoTs (Healing over Time) up, for if you get knocked back or crowd controlled, you might not make it to save your team mates. A good HoT skill is Rapid Regeneration.
*Ressurect a team member, if you have the time for it - if not, continue keeping the remaining alive.
*If you are templar, slot Luminous Shards and Restoring Aura. Use Luminous Shards at the tank, when you fight during bosses, it will help them regain stamina & magicka and continue blocking. Use Restoring Aura during mob fights, this way you provide your whole team with stamina regen.

If you play as a Dps:
*Your main job is to deal damage. Don't wear tank gear, you'll be dealing 2k dps when you should be dealing much more.
*Slot AoE (Area of Effect) skills for mob fights, slot single target damage skills for boss fights.
*If you are bow user, keep yourself within 28 meters radius of the healer. If the boss comes at you, don't run away or els you will be resetting the boss. You can not avoid getting damage in a veteran dungeon, so L2P in a group!
*If you are magicka based, while you deal AoE try to block-cast your skills. You block-cast by holding the right mouse botton down, and press the 1, 2, 3, 4 or 5 button to use the AoE skill. Remember that you are wearing lightarmor, you are squishy.
*If a team member dies, try to ressurect them. The tank must focus on the boss, while the healer is trying to keep you alive, so it is best if the dps that is alive ressurects.
*If 3 adds are hitting you, stop dps'ing the add that is 15 meters away from you, and kill the 3 close to you first! Use your common sense, because the healer is probably stressed and runs out of magicka if he has to heal you constantly.
*If you are melee Dps, get only close enough to the boss till you can dps it, do not get closer than that. Many bosses have AoE around them, if you see red circle beneath the boss step out of it and stand close enough to the edge to be able to melee.
*Don't stand face2face with the boss, leave that for the tank and stand behind the boss. You as DPS are squishy.
*If you see the healer dealing with 3 adds, while healing the rest of you, go kill those adds first - if the healer dies, you all likely die.

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Disclaimer:
Dungeons can be done in many different ways, and I am aware of that. I wrote this guide, based on what I have experienced with new players and what they have struggled with. The purpose of this guide is to provide basic information, nothing more.