Is it possible to create customer groups in Quickbooks Online (Simple Start)?If yes, can anyone please guide me how this is done?

Welcome to the Community, hello.
 

Currently, it is not possible to create customer groups in QuickBooks Online (QBO) Simple Start. However, you can add a sub-customer under a parent customer. Please ensure that the parent customer is already set up. Here's how to create a sub-customer:
 

1. At the left navigation panel, click Sales.
2. Select Customer, then click New customer.
3. Enter the needed information in the Customer information window.
4. Mark the Is sub-customer box and enter the parent customer.
5. Click Save.

Additionally, if you plan to get paid in the future for the products and services you sell can make an invoice to your customer, check this article on how to do it: Create invoices in QuickBooks Online.
 

You can always get back to me here if you have any other questions about the customer. I'll be around to lend a hand. Have a lovely day.