Can you add additional contacts to customers?

I can think of two possible scenarios based on your concern, rochelle490. I'll discuss the details below.

You either want to add extra contact details for your customers or create sub-customers under your top-level ones. These options are possible in QuickBooks Online (QBO).

If you need to add multiple contact details, for example, email addresses to your customers, Edit their profile and use a comma (,) to separate them. Please see the screenshot below for your reference.

If you wish to add sub-customers under your parent customers to track individual ones who are also part of a group or the organisation, here's how to do so:

1. Go to Sales and select Customers.
2. Select New customer.
3. Enter the sub-customer’s name and contact info.
4. Tick the Is a sub-customer box and select the parent account in the Parent customer dropdown.
5. Tick the Bill parent customer box if you want to bill the customer with the parent.
6. Review each section and enter any info you need for the sub-customer.
7. Click Save.

For more details about organising customers in QBO, please see this article: Add and manage customers in QuickBooks Online.

Furthermore, I'd recommend visiting the Sales page for a more comprehensive overview of your sales transactions and invoices. To know more about this, you can check out this article: View sales transactions in QuickBooks Online.

Please know that I'm just a reply away if you need further assistance managing customers in QBO. Take care, and have a good one.