amateur formation
I had an opportunity to fly my aircraft in a 4 ship formation at Sun 'n Fun 8 years ago for a photo shoot.
I had around 600h TT at the time and was very familiar with the aircraft but deferred the formation part of the flight to a very experienced ex-military training pilot who kept a very tight formation far better and more safely than I could ever have hoped to. The experience was one of the most exhilarating I have ever had in an aircraft and I can really see the thrill of it as well as the practical advantages it offers.
A few months before this I had attempted to formate with a far slower aircraft being flown by 2 good friends (one of whom was an airline pilot) up in Canada. It very nearly ended in disaster - I came very close to hitting them and killing all of us as I attempted to formate on them but bowled past them trying to slow down (at double their cruise speed). Things can happen very fast when you are close together, so unless you are trained I would recommend a healthy degree of separation.
IMHO It is a very good way to kill yourself and take a friend with you unless you have been properly trained.
SB