When editing a menu link it is possible to link to nodes from other groups

I implemented it by creating an entity reference view that filters by group argument and then altering the link widget on the menu_link_content form to use that view for autocomplete. But yes it would be a helpful feature! +1

You can also use group_sites.module and possibly a custom group context. That's how I would recommend doing this. We are currently doing just that sort of thing, with group_context_path_prefix. Marking as postponed, since those other contrib modules might solve this problem without any new features here.