Can any doc tell me what's the best gym class for a person who isn't fat but just needs muscle?

Muscle mass: The best way to increase your muscle mass is resistance training. this can be done with your own body weight, rubber bands, or traditional weight lifting. As to what class at the gym? I would look for weight lifting to maximize your muscle mass increase. However, make sure you fully understand the equipment before using them. Hope this helps.

A balanced approach: to exercise, including aerobic/cardiovascular work, is probably best for overall health, but for specific muscle-building purposes, progressive resistance training is the most effective. There are many options, including weightlifting with barbells/dumbbells, machine circuits like Nautilus or Cybex, kettlebells, etc... Talk to the trainer at the gym about what would best fit your goals.

Resistance & protein: Resistance training very important. Lower reps build strength more but burn less calories than high reps. Get at least 1 gram of protein for each two pounds of body weight. Keep hydrated!