I'm personally trying the reg park system (pre steroids style powerlifting, through stronglifts.com) because i'm aiming for a bit of bulk (i have no illusions of becoming mahoosive).
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.
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.