Groups are the way to help players level up and defeat bosses quickly. Though all the classes by themselves are very good in one skill and bad in the other, in a group, they all compliment each other. Dragonknights can take hits, Spellweavers and Steam Mechanicus can deal plenty of damage, and Rangers can be a sub for either. That is why every class is valuable in a group.
However it should be noted that Group members share all the experience points they get. They ALL share loot and experience.
In certain areas, namely parallel worlds, events and destructor, the monsters health will scale according to how many people there are in that group. Groups can still be beneficial as armor break can stack up to 2 times. A spellweaver's Singularity, a steam mechanicus's C14 Micro-Rocket, and a dragon knight's Iron brow, stack (up to 2 times) such that monsters can have -170% armor. Two debuffs of the same class will not stack however; a monster that has two singularity's cast will only have -85% armor, and not 170%. Each class has stuns that help with crowd control.
Not long ago, Parallel World dungeons were changed. Increased difficulty levels were added, which require increasingly Stronger Groups for each one of these levels. According to BP, more of these levels will be added in the future.