Another gotcha for you:
SharePoint Online has a great feature that enables your company to interact with external users, to better collaborate, without having to go through the whole process of adding user accounts, granting permissions, defining groups etc.. (not that you cannot do this of course, if you would like to take the matter into your own hands)
Now what the document does not clearly states is WHAT exactly CAN be shared EXTERNALLY?.
You can infer the security scopes available for sharing by looking at the table “Deciding how to share”.
As such, the share-able items are (read security scopes):
Site (Web), List Items (Documents)
As such, you will not be able to share a List, but you will be able to share a document library (it’s root folder).
You will however be able to share List Items.