I mentioned in the other thread that in school I was in the "middle tier"--not popular, but not an outcast either. I was notable for getting good grades but otherwise I kept my head down, stuck to my own friends (whom I wasn't necessarily that close to, but they were the "least non-compatible" kids in my small school), and wasn't interested in doing social stuff anyway. It seems kind of awful to say, "Keep your head down, don't try to reach beyond your circle." But, thinking back, a number of the kids I saw who were bullied or ostracized (especially girls) were the ones who really, really wanted to be in the popular groups, and made themselves clingy, embarrassing pests by hanging around the popular girls whenever they could.
So I would say, if you want to be friends with someone/a group, ask yourself why first. If it's just because they're popular and pretty and throw cool parties--that's not a good basis for a friendship. (Or if you keep trying to "befriend" a popular boy because you think he's cute and cool, without knowing anything else about him.) Find people you have something in common with first--you see them reading a book you like or wearing a shirt for a band you like, or you're both in a club together. Kids can smell desperation a mile away, it's like blood to a shark.