You and your friends may have to get creative.
http://www.nintendoworldreport.com/news/38853/super-smash-bros-for-wii-u-8-player-control-options-detailed
Nintendo has revealed the full list of compatible control options for Super Smash Bros. for Wii U's recently unveiled 8-player mode.
In the event that players don't have 8 GameCube controllers lying around, the GamePad can be used as a control option as well; although it's worth noting that the game will only support the use of one tablet. Additionally, the brawler's massive mode will support up to 7 Wii Remote or Wii Remote Plus controllers, which includes support for the Nunchuck and Classic Controller Pro attachments. Likewise, a maximum of 7 Wii U Pro controllers can be used.
Smash fans hardcore enough to pick up two GameCube controller adapters will be able to plug in and use all 8 of those slots, and, finally, those with a 3DS and copy of the handheld version of Smash Bros. will be able to use that as yet another control option. Up to 8 3DS devices can be used at once so long as each system has a copy of Super Smash Bros. for 3DS.
Super Smash Bros. for Wii U launches on November 21, exclusively for Wii U. Let us know which control option you plan on going with in the comments below.