I have done a few cycles of p90x. I have always been on the slender side but I went from 140 lbs to 160 lbs by complimenting the p90x videos with Total Gym workouts (ya, the Chuck Norris machine). P90x is great for losing weight and toning, but is not geared towards gaining mass which is what I was after. So I integrated the Total Gym routines to compensate. They have a forum on beachbody.com that talks all about it. You should check it out. Here is the link with answers to a lot questions people have about p90, p90x, and p90x PLUS.
Yes a pull-up bar is essential as it is involved multiple times is every week (except the "rest weeks"). Other than that, all you really need is dumbells or bands and your all set.