Thanks for joining us here in the Community, addisonleann.
I'd be glad to lend a hand so you're able to enter your customer's email address for billing in QuickBooks.
So I'm able to give you the best info, are you currently using QuickBooks Online or Desktop? If you're using the Online version, you can enter multiple email addresses in the customer's profile. Here's how:
1. Click the Customers tab in the left navigation panel.
2. Select the customer's name from the list or select the New customer button in the top right corner.
3. If you're adding an email to an existing profile, click Edit after selecting the customer's name.
4. Enter an additional email address to the email field, separated by a comma.
5. If you're entering a new customer profile, you can do the same with multiple email addresses in the same field.
When you're ready to invoice the customer, all of the email addresses entered will appear on the form. You can see what this will look like in the screenshot below.

You may also consider using the parent/sub customer option. This gives you the ability to create sub-customers under one parent customer. With that, you'll be able to select to bill the parent instead when it comes time to send an invoice. The following article provides additional info about using the bill parent customer option on invoices in QuickBooks Online.
I hope this info helps! Please leave a comment below if you have any other questions. I'll be here to help.