We are following the On Demand Migration Admin Guide documentation to populate a collection using "Fill from File" using the UserPrincipalNames (UPNs) in a csv file with a single column for UPN. It is guest accounts we are attempting to populate. That did not work.
Contrary to what the docs state, we were able to populate the collection using the ObjectIDs of the Guest objects by using a CSV file with a single column for ObjectIDs.
That section of ODM Admin Guide doc currently says that "one of columns must contain case insensitive user principal name data that will be used for matching". We have just proved that is not so.
The 'Populating Collections From a File' section of the ODM Admin Guide doc ought to have that section amended to say that users can populate a collection of Guest objects using a single column CSV file of ObjectIDs.