Just put this device into your arena map or anything like that! It will sort every player in a certain room into two same sized teams! Awesome features include:
*Arguing about teams? Just scramble them!
*Every team will have the same number of players
*The machine will handle setting up the teams for you!

Important: Please import the schematic with MCEdit. Worldedit sometimes bugs the machine and it will randomly teleport people to their death.