However this setting won't be exported into the add-on's XML without warnings. A new additional group relation should be exported also, even for default Xenforo options.
I tried to circumvent this issue and recreated the default Xenforo option but with a new additional group relation in my XML. This works, but the default Xenforo option does get associated with my add-on silently and therefore would be deleted if the add-on gets deinstalled. This should not happen.
It looks like you automatically assume the add-on id in the parent node of the add-on XML for all its child nodes. BTW, this is just the opposite you do with language XMLs. ;-)