Seconding what the others have said (and as a fellow Crossfitter, I completely agree with what Outdoors has to say), I'm going to get very detailed about what worked for me. I saw huge differences when I did three things:
1. EAT, EAT, EAT. I'm a very experienced lifter a with slavish dedication to the gym and rugby. It was/is not unusual for me to work out 2x, sometimes 3x a day (morning (Crossfit) and lunch (strength training) at the gym every day, then at rugby practice at night 2x a week). So I my high metabolism was due to my high physical demands. I struggled for years to gain muscle until I hired a trainer who dialed in my nutrition and DOUBLED my then normal caloric intake of 2100 to close to 4500 calories a day. (I was 34 yo, 5'7' and then was 165 lbs) I literally was eating every two hours. I thought for sure I'd get fat, but took trusted that my trainer (who had the kind of build I was looking to have) knew what he was doing.
And guess what? Twelve weeks later I had gained 13 lbs of pure muscle AND my bodyfat had decreased to 15% from 17%. All natural, thanks. By week 20 I had gained at total of 22 lbs of muscle from the time I started with John (my trainer). Folks thought I was on the juice, but I wasn't (never have been). After years of training intensely, my body was primed to grow. I just needed to eat enough so that it could.
That was two years ago. I'm 36, still 5'7" (damn!), but now 192 lbs, 12.7% body fat, eating 3200 calories a day (maintenance).
Was it easy to eat that much all the time? Hell, no! But, like my muscles used to be, your muscles are STARVING for fuel. Pretty simple: if they don't get enough food, they don't grow. You lose weight. If they get enough food, they grow. You gain weight. Get your eating dialed in. EATING IS YOUR #1 PRIORITY IF YOU WANT TO GAIN MUSCLE. 80% of building muscle is all about the muscles getting the nutrition they need to repair and grow.
CAUTION: You need to ramp up you caloric intake slowly, over a couple of weeks. At 5'11, your normal caloric needs are probably around 2400 calories. If you're used to eating 2400 calories and all of a sudden jump up to 4000, it will shock your body, and not in a good way. Add 200-400 calories to what you're eating now each day, until you reach 4000. Then watch what your body does over the next week. If you don't gain weight (i.e half a pound), then add another 500 calories to your daily.
Also would recommend supplementing with glutamine. You deplete your body's glutamine stores with high-intensity work. Without supplementing glutamine, you slow down your recovery rate, and you can lift as intensely as you did at your previous workout. I recommend Optimum Nutrition's Gold Standard Whey protein, which contains a proper dose of glutamine in every serving, as well as the coveted BCAAs.
2. HIGH INTENSITY, HIGH WEIGHT, 5 -10 REPS, at least 3 sets, focused on COMPOUND movements. High intensity, heavy weight produce a greater hormonal response. You need to shock your body into producing growth hormone at optimal rates when you sleep. High intensity workouts are the only way to do this. Rep ranges of 7-10 tend to promote hypertrophy (bigger muscle) more than strength. But don't worry, you'll still get plenty of strength development. You need to lift as heavy as you can, so aim for at least 5 reps, and go for up to 10 if you can.
Use the barbell and dumbbells mostly and focus on compound movements (i.e, squats, bench press, pull ups and bent-over rows, Military press). YOU MUST DO SQUATS, free standing squats (no hack squat machine, Smith machine or leg press). Three, intense sets of heavy squats alone will elicit the shock/hormonal response you're looking for. You want to be almost dizzy at the end of each set. Barbells and dumbbells recruit stabilizer muscles, which further shocks your system. Compound movements use more groups of muscle and promote the fastest gains. The bench press, overhead press and pull ups will work your arms plenty, but you can throw in some curls and skull krushers if you want. If you must use a machine (say, if you don't have a trainer or a spotter), try to use the Hammersmith machines. Stay away from the pulley machines if you're trying to gain muscle. They restrict your range of motion too much.
For your purposes, 3x a week at the gym is enough. You should be adding five pounds to your lifts every time you hit the gym. For example, if you squat 200 on Monday for 3x5-10 and get every rep, you should squat 205 on Wednesday for 3x5-10. If you get all the reps on Wednesday, you should squat 210 on Friday for 3x5-10. Let's say you don't get all of the reps on Friday. Then Monday, dial it back 15lbs (195) 3x5-10 reps and try the progress again. You want to CONSTANTLY test the limits of your abilities every time you hit the gym,to the point of puking. This is what stimulates your body's hormonal response when you sleep at night, as it adapts to the intense physical demands you are placing on it.
This protocol is laid out in detail in _Starting Strength_ by Mark Ripptoe and Lon Kilgore, considered one of the weight training Bibles. (Where else are you gonna find a book that spends 50 pages just on teach the squat!) Many, many hard gainers have put on mass by following this protocol (and EATING shitloads). Highly recommend getting this book, and his other book, _Practical Programming_. Ripptoe lays out the very protocal my trainer used with me over the past two years, so you can save yourself a few grand if you can't afford a trainer.
3. SLEEP, SLEEP, SLEEP. Your body grows when you are at rest. This is when growth hormone is released in its highest concentration. You want to get two good deep sleep cycles in, so (1) go to bed early and at a regular time every night (this includes weekends) and (2) sleep in as dark a room as possible (pitch black, if possible), to promote the deepest sleep cycles possible. Get at least 8 hours. If you're working out as intensely as you need to, you will have no trouble at all sleeping! LOL Make sure you down a protein shake an hour before you go to sleep, so that your body can use those nutrients to build muscle during the night.
Be patient and dedicated. A half pound gain a week is GREAT progress for a hardgainer. There will be weeks where you don't grow at all (or even lose weight). But those weeks will be balanced out by weeks where you gain 1, 2 or even 3 lbs of muscle. Give your body the 4-6 weeks it needs to adjust to the "new normal" of intense workouts and high caloric intakes. You'll start to see real changes by week 6. People will start to notice those changes by week 8. 10 lbs of muscle may not sound like much, but you will be amazed at what gaining 10 lbs of muscle looks like on your body. It looks like so much more than 10 lbs!
This approached worked for me, and for all of the other guys my trainer trains. Hopefully there are some useful nuggets here you can apply that will help you out.