Not every kid fits neatly into a category. There are basically three types of "outsider" groups that a teen might fall into. The socially challenged teen has trouble making friends and just doesn't fit. The independent teen might be a "floater," who has a variety of friends and feels no need to join any one group exclusively. The determined outsider actively rejects cliques and may even hang with fellow outsiders, who form their own group (go figure).
Experimenting is natural during the teen years, and kids often change their circle of friends. Some teens really are loners by nature. It's important not to jump to the conclusion that there's something wrong with them.
At the same time, outsiders can become the targets of bullying, so keep an eye on these teens and be ready to intervene if they become too isolated.