The PMPG was established in 2001 by a group of buskers in Pike Place Market. They bring a collective voice to interact with the PDA (preservation and development authority) which governs the market. They also produce an annual festival which will happen September 14th, 2008.
This link takes you to the Pike Place Market website where they give the scoop on how to become a performer there.
Busker World is based out of Australia. Run by Johnie Mac, a fifteen year busking veteran, who has played all over the world. Definitely worth a look.