This event is hosted in Trackmania and in the title(s) Trackmania² Valley. You can participate on your own and registration is not required. And there is a map pool that you can contribute to.
The Mania Exchange Knockout is an online competition where the last player to finish each map is knocked out until the winner remains. You can read the full rules here and a more detailed explanation of how the competition works.
New maps are always welcome. See the map building thread and the #knockouts channel for more information.