Hi alarmone,
Our Help Center is the best place to go for instructions. Here is a link to creating Customers. For info on Field Services, click this link. You'll also probably want to check out the Work Order article and the Creating Invoices article.
For invoicing, I'd recommend creating the work order for the customer (child or job in QuickBooks). Then, if you bill the main or major company, you can create an invoice from the work order, and when you're taken to the invoice screen, you can change the Customer for the invoice to the main company. This way the main company is invoiced, but the work order and time entries are logged for the sub or child customer.
Hope this helps,
- Adam