Last I knew the biggest duelling guild on PC EU was called Colosseum.
The players at that duelling spot and in that guild tend to be fairly hardcore. If you are new to duelling, I recommend practicing with friends, although I assume the guild has people willing to train and give advice.
PC EU, Stamina DK (EP), Magicka NB (EP), Magicka Warden (EP), Magicka Templar (EP). Occasionally: Stamina Templar (EP), Stamina NB (AD), Stamina Sorcerer (DC)