You need to finish every level with your friend!
11.8.2014 update v0.1.2
For more info check my wiki down...
GameMaker Server is a GameMaker extension which allows you to add online INIs, online achievements, online highscores or online multiplayer to your game, without having to host your own server.
There are 1,019 developers using GameMaker Server, and 4,958 accounts have been registered. GameMaker Server hosts 1,280 games, and 124,152 logins have been made on 34,939 different usernames.
To use GameMaker Server, you'll need a GameMaker Server account, and you'll need to register as a developer. Every game has to be registered as well, before any of the online functions can be used.