When creating an organization chart, you don’t need to include everyone in the organisation. Obviously you’ll include those that are in the in-scope parts of the business system. You’ll also include appropriate people from the parts of the organisation that are out of scope, but who receive or give data to the business system that is in scope.
There are no hard and fast rules here, do what paints the right picture and guides you / your team in working out who to interview etc.
The project manager and stakeholders can help. In a large organisation, Human Resources / Personnel should have an up-to-date (or close to) organisation chart, to form a basis for yours. If you are preparing an Org Chart with many people, you may only want to name team leaders or senior managers and above.