The game has a decent amount of group content most of which can be done from either level 10 or 15, forgot which.
The game is essentially split like:
Questing and Overland zone content: Solo
Delves (4-6 per zone): These are mini public dungeons (1 Skyshard and 1 Boss, sometimes a quest): Solo
Public Dungeons(1 or2 per zone): These are similar to Delves but larger and harder ( 1 Skyshard, 4-5 Bosses and 1 Group event): Group (But can be soloed. some boss can be challenging solo)
Instanced Dungeons(1 or 2 per zone): These are traditional dungeons, require 4 people. Original game are fairly easy with some soloable or done with 4 dps, DLC ones are much harder and require tank/healer and 2 dps. These are available in Normal, Veteran and Veteran Hard Mode for some(which I think are for the last boss only).
Trials: These are raids intended for 12 people, available in Normal and Veteran difficuties.
World Bosses(4 to 6 per zone): These are group boss which require a group, some can be soloed by well geared players but they are pretty tough. Like dungeons the World Boss in the original game are significantly easier than those from DLC and Expansions.
Maelstorm Arena: Solo instance where you defeat monsters. 9 stages of 4 Rounds each. Has normal and Veteran mode.
Dragonstar Arena: 4 man instance. 10 Stages, and has Normal and Veteran mode.
Blackrose Prison Arena: 4 man instance. I think it's 5 stage of 4 rounds each. Available in Normal and Veteran.
There's also "events" in each zone, which scale to number of people. There's like 3-5 spot in each map where these happens semi-randomly. They are pretty straightforward, kill waves of mobs and finally a boss.
Most of this scales to your level, so you can do pretty much anything at any level which is pretty cool. Obviously high level player with better gear and champions are stronger but still doable.
The thing with ESO is that for the most part just above would be an apt description. Some of the quest are really interesting, some are pretty bad. Combat has some cool moments, but for the most part it's not great.
This is actually my 3rd or 4th attempt at the game, never made it past 27 before. I finally stuck with it this time and I'm level 50 with 293 Champion Points and enjoying myself quite a bit. I guess I got used to the unspectacular nature of the game. It has tons of content, and exploring zones, and doing zone completions is super fun for me. I got 100% completion in GW2 so it's something I just enjoy doing.
There is tons of content for a casual player like me. There's 30+ zones from smaller starting area to some being pretty large, over 35+ dungeons. I'm about halfway done exploring the zones only. I have done a few dungeons, no veteran mode yet though and no trials. I completed my first Maelstrom Arena a few days ago.
I really wish the combat was better, and game suffers from poor optimization on client and server side. It's extremely playable but it should run better. I heard there's a lot of issue in Cyrodil which is the open world PVP zone which castle siege type gameplay.