Existing 'Users' custom field should include portal users as we..

Currently the 'Users' custom field only exposes internal users. For our private Ideas portal, the user who creates an Idea for a given Organization is not always the same user who has final approval if the scope/requirements. If we can capture the Approver in the User field we'll have a much better workflow and collaboration experience as a team. This could also be used in collaboration with previous ideas around being able to assign To-Do's to external portal users.