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.