Microsoft To Do supports list sharing for the following scenarios:
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Sharing between personal Microsoft accounts.
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Sharing between accounts within the same place of work or education.
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Sharing from personal Microsoft accounts to work accounts, as long as an organization’s admin has turned this feature on. (Note: it's currently not possible to share from work accounts to personal Microsoft accounts.)
There are a few reasons why you may not be able to join a shared list:
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If the list owner deleted a list before you had a chance to join it, you'll see an error message advising you to contact the person who shared the list with you.
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If a list is owned by someone with a work or school account, that list will be unavailable to anyone outside the owner's place of work or education.
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Work or school accounts can join lists owned by personal accounts, as long as the work or school's IT admin has turned this feature on for their organization. However, personal accounts can't join lists owned by work or school accounts.