Open Customers and press Add customer (or add one while booking with New customer). Enter the Business name, business number (ABN, EIN, BN or GSTIN; a wrong format is refused), Payment terms (days), Accounts email (where invoices are sent), Phone and Billing address, then save.
The customer can then be chosen on bookings, gets invoices and can have their own rate card. Press Portal next to them to give their staff a portal login.
FleetKraft keeps one record per business: a business number another customer already has is refused, and a customer with the same name is only added after you press Add anyway, so jobs and invoices are not split across two records.