I've always been quite skinny and my metabolism drives it very well, I sweat fast, etc. And I'm tall too, which means more muscle mass to get the same size. I also did the P90X doubles version, which starts at month 2 and adds 3 more endurance workouts weekly. All that taken into account, I still managed to get good functional muscle growth.

It is very easy to modify the program to suit your needs. If your friends do not care that much about endurance, I would suggest not doing the doubles version, keeping rep counts low (eight or less) and trying to increase the weight from week to week. They should also take in enough protein, perhaps through whey protein supplements, but definitely no steroids.

What's more important than anything of the above is how much effort you put into every workout. If they push themselves hard enough during the resistance training parts, there will be nothing to worry about and there's plenty of massive after-pictures from P90X online to prove this.

P90X will give you massive performance improvements for almost every physical activity. If you just want to bulk up, look up hypertrophy training. That will be much easier mentally if you don't necessarily care about fitness.

I've seen a few hundred P90X before/after shots from real people and didn't see much size growth. Skinny people got cut, which makes them look bigger and fitter. Doesn't mean they are heavier when they weigh in. Maybe added 5-10 lbs of muscle in a year. Full people got cut, also looks bigger and fitter in most cases. When you work out the whole body your bodyshape changes, shoulders get bigger so the perception is that you are bigger

A lot of hollywood actor training uses these principles. Actors need to get in shape quickly for various roles and trainers focus on getting the ripped P90X look instead of forcing real growth in such short timeframes. Ripped will look better naked than adding 10 lbs of non-ripped muscle. For example Bradd Pitt was maybe 160 lbs in Fight Club but looked much bigger because of muscle definition and upper body shape.

But if you really want to go from skinny to big/normal, not just ripped, heavy weights are the long term solution. 30 lbs of new muscle will look better than ripped. I don't know the details of P90X but recommended equipment is 50-90lb dumbells and a pullup bar. Most people use 50 lbs max and that's not going to cut it.

That's probably true. As a rower, I didn't need to be much heavier, just stronger. I put on about 4kg (almost 10 lbs) during those 3 months not even counting the fat I burned. And I didn't have the option to increase weights much, didn't eat that much either.

Depends on their calorie intake mostly. If there's a calorie deficit (they burn more calories than they eat) they'll lose weight and look like rakes. If they eat more than they burn, they'll get bigger and usually stronger.