Suppose a group of people share small segments of a copyrighted work in a chat room. If the song "Amazing Grace" were still under copyright, it could go something like this:
<A> Amazing grace... <B> ...how sweet... <C> ...the sound... <D> ...that saved... [and so on] <Y> ...but now... <Z> ...I see.
If each line is sent by a different person, have they collectively infringed copyright? They are deliberately posting a copyrighted song; they are not independently writing a song that happens to be the same as an existing one.