If a friend won't let you into a group - are they are a real friend?
Some friendship groups seem pretty flexible and welcome people to join in. Others seem much more restricted, though. People in these groups make it clear that not just anyone can be part of their crowd. That type of restricted group is sometimes called a clique.
Read more on this article here.
The friend must be feeling a great deal of pressure to be "cool" and "popular"! Though the article points out that that often comes at the price of the clique being extremely controlling.
There's some levelheaded advice on the last page for surviving inside or outside of a clique.