In the old days of lore, the groups went along the lines of jocks, geeks, stoners, etc. For quite a while, however, groups have been defined by taste in music. Sure, these kids had plenty of other interests and things in common, and there are groups of friends that are not limited by music tastes, but in general terms, music is what brings people together.
Music can serve as the instigator, the launch pad, for friendships.