Currently, the Calendar App only recognizes a contact list in the Task Owner field if the contact column is single-select. If the column is multi-select, the app returns an error when trying to enter information in this field from the "New Item" function and does not let you edit if there are already contacts in the field.
It would be great if the app recognized multi-select contact columns as well so that the full functionality of the app can be used. Currently, this information needs to be edited/added on the source, outside of the app itself.