This is an MMO game and playing in group is more fun and more rewarding. When you are in group and someone in group kill something or earn XP than you will also earn XP even if you dont attack and even if you don't kill. Accept those group invites, especially when in dungeon, grinding or just leveling. Whole group will gain more XP, there is no stealing of kills.
It will be rewarding for whole group.
Thanks for reading