Especially early in the game, do not expect to attack a mob of enemies by wading in and taking them down Group style the way you did in TW1. It doesn't work. It's a Good Thing that it doesn't work anymore. In this game, enemies are fast, well armed, and deadly, and they do not stand around waiting to be hit.
Since nobody is watching your back, you must keep enemies in front of you and engage as few of them at a time as possible. This is especially important at early levels when the skills to mitigate backstab damage (Position), parry and counter effectively (Riposte), and roll for distance (Footwork) are not open yet. Signs, especially Aard and Yrden, but also Axii when you can get away with it, are effective at slowing down and breaking up crowds. Kiting and using the obstacles in each area are also useful. You can break up a crowd just by running away from it, then turning and picking off the fastest enemy.