From Venture Beat:
When you start an online match of Mario Kart 8, you get an option to either go play with the original 32 courses or with all 40 courses (the original 32 plus the eight DLC tracks). Likewise, when you create a tournament, you can select 32 or 40 courses, but you can also select to only play the DLC levels.
Of course, if you make a 40 course tournament, everyone that wants to join will need the DLC.
But you will never come across a situation where someone will get kicked out of a lobby because they don’t have the new content. People with the DLC will also never have any disruption to their experience because other people didn’t get the new courses.
http://venturebeat.com/2014/11/06/heres-how-mario-kart-8s-downloadable-content-works-online/
Basically those with the DLC have a new online mode to play with that lets you choose all 40 courses. Hopefully there will be a lot of people who choose this so the lobby won't be empty!
