there's a few conflicting ideas going on here. mostly, if you want to add muscle, there's a good chance you're going to add weight. the first thing you need to do is tell your family that you're not too skinny, everybody else is too fat (i seriously have to remind people that while i am a lot thinner than i used to be, i'm still a little overweight...when did overweight become the new standard?); then you need to throw out your scale. mostly, if you are interested in building muscle, you need to do resistance work. i think Primal Blueprint Fitness is perfect for this...all bodyweight, minimal equipment, and it can get hard. if you don't want to add a lot of size, but want to get and look strong, PBF might be perfect for you.