Hello,
sites are "just" normal content objects. You could provide a custom aspect that links / groups sites, apply it to a site with a custom action (or via the Repository view) and develop a dashlet upon that data to display "related sites" / "sites in group" on the dashboard of each site.
Unfortunately, there is no out of the box functionality for this as each customer / user has specific requirements and a generic approach in the product would usually not be sufficient…
Regards
Axel