Now that you know there are several ways to instantiate MVC groups we can go back to customizing them.

The simples way is to pass in new values as part of the arguments map that mvcGroupInit receives, for example

def group = app.mvcGroupManager.buildMVCGroup('foo', [key: 'foo'])

However is you wish to use the special config key that every MVC group configuration may have then you must instantiate the group in the following way

def configuration = app.mvcGroupManager.cloneMVCConfiguration('foo', [key: 'someValue'])
def group = configuration.create()

Note that you can still send custom arguments to the create() method.