Keychain sharing not working with app extension

I am currently trying to access the last logged in user of the main app in an app extension. It seems that although keychain sharing is enabled in both and the main app and extension are in the same app group, the extension does not access the same keychain to restore the session.
I first found this out when I implemented the login via GoogleSignIn.
The function `restorePrevoiusSignIn()` works in the Main App as expected but not in the App Extension.
Now that I have checked the source code of GTMAppAuth, it seems that the AccessGroup is not defined anywhere (and if so, where can it be defined)?