AngularJS: How do I create controllers in multiple files

I'm trying to split my controllers into multiple files, but when i try to register them at my module im getting an error:

groupcontroller.coffee

app = angular.module('WebChat', []);
app.controller 'GroupController', ($scope) -> 

usercontroller.coffee

app = angular.module('WebChat', []);
app.controller 'UserController', ($scope) -> 

Error

Error: Argument 'GroupController' is not a function, got undefined

From the documentation I dont really get what the module method does anyway. Does it store my controller with the key 'Webchat'?

Edit: It also seems that passing [] creates a new module and overwrites the previous one

app = angular.module('WebChat', []);

To prevent this, you have to leave out the [] like

app = angular.module('WebChat');