game is basically solo play until lvl 19, this is when you enter your first dungeon(assuming your keeping up on your quests).
from 19-40 you can solo or squad for your quests but there are not many dungeon type quests.
When you reach lvl 40 you get daily quests in dungeons which makes the game much funner.
at lvl 60 you can start doing a variety of dungeons for profit and gathering mats for elite gear.
It takes a while to get to lvl 60 but the game completely changes at this point, and makes solo questing almost impossible for some classes.
If you want to play in squads a lot, pick a barb or a cleric. These are the 2 classes NEEDED for all dungeons.
Every other class although important, is not needed in particular except to fill the squad for damage.
The game is rather basic, whoever does most damage holds aggression(except for skills that create more "hate")
Barbs need mass hp and to be up to date on aggression skills so they can tank every mob that is aggroed, and keep everyone from being hit.
clerics need mass magic and mp to keep the barb alive.
everyone else stats on however they can deal the most dmg.(until lvl 80)
At lvl 80 if you choose to, you can compete in territory wars with factions. some DPS,DD add more into hp so they can stay alive longer since players dont go by aggro.