You will definitely start to get stronger in the upper body, but I wouldn't join one specifically for losing weight, if you have the self desire to do so as the exercises aren't revolutionary. Basically you need a calorie deficit (500 deficit a day will lose you a pound a week), so if you like your food too much increase your training load. An hour's boxing session is equivalent of a 9km run in energy expenditure terms, which is around 600-700 calories. So find yourself a calorie counter online, take off 500 calories and aim for that while staying active.