Each client can have multiple contacts. Those are stakeholders (PMOs, sponsors, vendors, etc.) that need to be aware of the project progress.
In order to add a new one, you need to go to the desired client in the Clients section → Contacts → Add contact.
These contacts can be used when sending estimates, invoices, or late payment reminders. You can also add notes and grant them access to the client portal.
NOTE: Only admins can add contacts.
NOTE: If you've marked someone as the main contact for the client, you'll notice them after the green highlight around their contact.