Open collaborative design/Who contributes?
Anyone who has the desire can contribute. In practice, collaborative projects usually attract a core of dedicated, passionate amateurs with an interest in the relevant subject. Everyone (or nearly everyone) is passionate about something, whether rock climbing or chess or music or architecture or physics — open collaboration harnesses this enthusiasm and uses it to build something for the good of all. The Internet allows a person to find others who share their passion and network with them.
The mountain bike, for instance, was developed by a group of enthusiastic cyclists who were dissatisfied with mass-produced racing bikes. Because they were users, they knew what they wanted and were able to test their designs in the field. New forms of music have always been developed in this way.
Charles Leadbeater has put forward convincing evidence that innovation and design are being driven increasingly by groups of dedicated and highly skilled amateurs, whom he calls professional amateurs. See his TED talk or his essay for more.