In other words, with positive peer pressure, you observe your social surroundings and realize that something you had thought was forbidden is actually acceptable (at least to some people). What's interesting is that positive peer pressure only matters if it was something you were thinking about anyway. For example: I get lots of peer pressure telling me it's okay to go to a sports bar, spend lots of money on alcohol and bad hot wings, and watch hours and hours of college basketball. But since I have no inclination whatsoever to do that, the peer pressure is totally irrelevant. Likewise, when I was in high school, I experienced a lot of positive peer pressure with regard to various drugs. I never smoked any pot, but many of my friends did, and that was okay. I did get drunk once (while underage, gasp!), and that was okay, too.
Negative peer pressure, on the other hand, tells you that you must do something you'd rather not do or face social disapproval. You have to do something or you won't be socially accepted. Now, this shows up all over the place in very mild form having to do with "what's cool" in clothing or entertainment or whatever, but the really significant instance that springs to mind has to do with religion. I remember attending church (mormon) in middle school with my dad's family and feeling compelled to get up during fast and testimony meeting and say some nice things about the church. I did it with lots of mental caveats and unspoken reservations, but I did it because it was clear that that's what was expected.
It seems to me that whether positive peer pressure is good or bad depends entirely on the thing it's giving permission for. Negative peer pressure, on the other hand, is inherently a little bit bad, because it's coercive in nature. That can be counteracted by the moral quality of the action it's encouraging, but the negative peer pressure itself is, well, negative. For me, that's an interesting refinement of the messages we all got in school in the 80s, which was basically All Peer Pressure Is Bad (AndDon'tDoDrugsBecauseThey'reReallyBad)
I thought that was an interesting distinction, and I don't believe I've ever seen it made before.