To answer a couple of your questions
you don't need to file a flight plan
Airtraffic seem to like it. They get rid of 2 or 3 aircraft and they only have to talk to one of them. They more or less ignore the rest of the formation.
And to echo what everyone else has said, don't try it without training. It is fabulous fun. Formation Aeros is terrific fun and dog fighting is even better. You'll never want to fly alone again. (Oddly also the hardest place to fly in a formation is number 1.)