For anyone experiencing the same problem, I think I have found a work-around.
I am not an expert on IMAP or Dovecot, so please forgive me if I have used incorrect terminology.
It appears that GO requires that "root" folders to have no parent. This is intuitively true for private folders but is not true for shared folders (at least for Dovecot IMAP).
To follow my previous example....
"Accounts" is a child of "Company" but for shared folders "Company" is not a folder but a namespace. In the Dovecot Maildir this will appear as...
None of these folders are visible in Group-Office (even if they are subscribed) because they all have a "parent".
The workaround is to prefix all entries in the Company maildir with .Company as follows:
Because there is no ".Company" all the folders appear to be orphans, and GO treats these as root folders and displays them perfectly! Of course in any other IMAP client you will see an additional "Company" as the root folder like this....
... but at least the road warriors can access their email