Irock's right, there'd probably be mainly Nintendo people, but still, there'd be a lot of people testing, so a leak could easily get out.
WHERE!? IMAGE SEARCH SUPER SMASH BRAWL!
Outside feedback? Beta testing companies? Does a beta testing company exist? I don't think so, feel free to link me though. As Irock said, Nintendo can do most of that themselves. Besides, why would you need outside feedback on a sequel which has a proven core mechanic from two successful games?