The onboarding call should include the primary contact person who will be regularly interacting with countX and uploading data to the Client Portal. This should primarily be the individual responsible for these tasks, whether it's the CEO, another key team member, or a tax advisor if they will be receiving exports from countX.
Having a single main contact helps avoid confusion and ensures that important information is communicated clearly. If necessary, additional colleagues can be included in email communications to keep everyone informed.