I have found a way to make some progress with this, though it's not the full solution I would like it to be.
You can save a calendar from MS Outlook as an iCalendar file (.ics) which can then be imported into Thunderbird. You can only save the calendar if you have Outlook on a computer (I do as a dual boot ... but prefer to use Ubuntu mostly which is why I want the calendar in there).
Because this only imports a static calendar (ie it will not update if anyone changes the original calendar), I found it works best to import this as a separate calendar in Thunderbird, then I can delete it periodically and reimport the updated version whenever I want to update it. It works for me to do that about once a month but it's clearly not as good as a dynamically updating shared calendar.
1. To save the calendar from Outlook:
Open Outlook.
Click anywhere on the calendar you want to save (to shift the focus there)
Click on the File menu then 'Save calendar'.
It defaults to saving it as a .ics file.
You can - under 'More Options...' - select a date range to save, as well as what level of detail to save. Clearly, by selecting a limited date range, you will reduce the file size and saving (and subsequent import) times, and it probably reduces the risk of errors.
Save it to a USB stick.
2. To create a new calendar in Thunderbird:
Boot into Ubuntu.
Open Thunderbird
Click on the Calendar tab
Towards the left side, under the calendar title, it will list your current calendars. Right-click in the area below where your calendar(s) is / are listed and a select 'New Calendar' from the pop-up menu.
Click 'On My Computer' for where it will be located.
Give it a name, and a colour of your liking (I find it very helpful having the work calendar a different colour to my default one so I can see at a glance where the entries came from when I view both calendars at once). You can leave the email bit blank.
You will then have created a blank calendar.
3. To import the data into your new calendar:
Insert your USB with the saved calendar
In the Calendar tab, at the top right, click on the three horizontal bars (the hamburger)
Click on 'Events and Tasks'
Click on 'Import'
Navigate to your USB drive and find the .ics file you saved - click open.
At the top, make sure you select the correct calendar to import it into - it may be fine to merge it with your current calendar, but in case of errors, it's nice to be able to delete it and reimport without affecting your other calendar's data. I did encounter errors the first time I tried to import it and just deleted the calendar and started again.
Click 'Import All' at the bottom.
That should then import all the selected data from the calendar that you wanted.
Please note as mentioned above, this calendar will be static not dynamic, ie no changes made on the original will update - so you'll have to do delete the imported calendar and re-import every time you want to update ... which is why I'd love for someone to solve this by showing a way to get the original sharing invitation to work on Thunderbird.
