I think Crossfit is great for beginners. Most of the exercises and workouts scale very nicely, so you don't need to be fit when you start. For example, a workout might call for 100 push-ups over the course of 20 minutes. If you can't do strict push-ups, you can do them on your knees. If that's still too hard, you can do push-ups with your hands on a raised surface or even against the wall. Similarly, if the recommended weight for something is 135 pounds, no one will look down on you for using 85 pounds or 25 pounds if that's where you're at. The important things are that you come consistently and work to the best of your ability. Everything else will take care of itself, regardless of your starting level.