Can anybody tell me how to create group of customers.
There is not a specific functionality. However you can group them through control accounts which divide them in different accounts under BS or through a custom field which can be used in some reporting.
I need a group for some customers of different cities but bill/invoice has to send to one main customer who will pay amount of all the invoices of his group member customers.
Do you issue only one invoice? What is the link between the main customer and all the others?
As @Davide wrote, you cannot create subsidiary customers under a main customer. But for each customer location, you can create a new customer with the same billing address but a different delivery address. Distinguish them by Name or Code.
You can create a custom field to sale invoice for cities. When creating the custom field check “ Show custom field as a column”.
You can then have export sale invoices to excel and sort according to cities.