Ted Ehara wrote:
There are also instructors like AikiWeb columnist George Ledyard, who have special classes and seminars dealing with multi-person attacks.
Mr. Ledyard has a cdrom for sale on the topic which is definitely on my to buy list. There is a review of his seminar somewhere on this site, I believe, which has a lot of great info on multiple attacks. I also recommend using the search function to see what he has had to say on the subject on this site.