Hi Sandra,

This can be done via these two API endpoints:

List Post Subscriptions: Lists the subscriptions to a given post.
List Topic Subscriptions: similarly lists the subscriptions to a given topic.

These will give you a list of Zendesk user IDs. To turn those into names, you'll need to use the Show Users API endpoint:
Show User

If you're not experienced at writing API scripts, Postman is a tool that can make the task somewhat easier: How do I set up Postman to make API requests to my Zendesk account?

Hope this helps!