The user ID field should be included as a column when generating a user CSV export in /analytics/export.
Most bulk user API actions require user IDs (e.g. https://api2-eu-west-1.insided.com/docs/user/#tag/Webhooks/operation/listEventWebhookSubscriptions). If the user ID field is present in the export it will be faster and easier to gather user IDs for bulk actions based on the CSV data.