Currently when no MediaWiki:Grouppage-x message exists (neither in language files nor in MediaWiki namespace) the group name is used as the group page on [[Special:ListGroupRights]]. That is a page in the main namespace. An example of this is the import group on English Wikipedia which links to [[Import]] but it is [[Wikipedia:Import]] that explains what importers do.
I think that for most wikis a page in the Project namespace is more useful. So the default group pages should be "Project:Groupname" (and not only "Groupname").