Occasionally, a default group for a SharePoint site collection can get deleted. When it does, it means that non-owners cannot invite others to the site and you might receive an error in places:
You'll most likely see this under Access request settings (Site Settings > Site Permissions > Access request settings in the ribbon).
Getting the default groups back is not very obvious but as it turns out, it is quite simple. Simply open the site in question and change the URL to:
This will open up the People and Groups page with the default groups:
Click the OK Button and the groups will be restored!