When it comes to tournament day just be sure to keep telling each player who joins the channel about your tournament / time of it. Try logging on as early as you can that morning letting people know.
I put it as the main channel greet for now.
If I can make it that day I will help out for sure. Most people will not sign up ahead of time or will want you to do it for them. That's why I just rather use forums for brackets then other sites, plus it gets more people here on the forums who can run into other topics and possibly sign up here.
Easy that way to just write down their names and throw it into a bracket , and add the link to your profile. If you want as many people as possible playing in it then that is probably your best bet without having your tournament start late due to people still needing to register.
You will run into a lot of guys who will want to keep playing in their current game instead of reporting to the channel for the start time. Be sure to give them deadlines before advancing their opponent so one of your brackets will not fall to far behind the other, causing even more afk's and making for a longer night and chances of people disappearing.