When designing the MSN Explorer application, the development team included several people joined together just for working on personas: 22 people.
The Windows Persona creation team consisted of 22 people: several technical writers, several usability engineers, four product planners, and two market researchers.
pruitt.pdf [PDF]
The PDF gives a detailed explanation on how they used personas, how they extended them. The interesting thing is that the personas created crosses team boundaries. Other teams adapted the personas, rewrote them and applied them again to their product.
The document also describes the way the persona team created the personas documents and how they communicated the research.