Organizing and accessing contact information in Outlook.com
A further update...
Still no joy with old groups. The ones that did reappear still only had a few of their members.
I can report, however, that the new "Lists" integrate beautifully with Office Outlook 2016.
Once you are on the NEW Outlook.com system (and it should happen in the very near future), [note that “Lists” replaces “Groups” as the terminology] you need to go into your Microsoft Office Outlook [I'm assuming Office 2016 or at least 2013], File>Account Settings>Account Settings and DELETE your outlook.com account, close Office Outlook, re-start it and File>Account Settings>Account Settings add your outlook.com as a new account. It will take a while to re-sync all your emails and contacts, but when you do, and you go in to People and change the View to List, slide across to find the Categories header and click on that, and lo and behold, your “Lists” appear as bold items, and clicking on them will reveal all the people you added to them.
Furthermore, you can now manage those lists from within Outlook 2016 – add people, remove people, remove Lists/Groups (not sure about creating new lists/groups). Double-click on one of those bold headings will open the list, and there is an email icon in the top bar of that list, which you can click to send an email to all its members.
As for those lost old groups.. I wouldn't hold my breath waiting for Microsoft to restore them.