How you do this would depend on why you are grouping the customers. Do you actually need them to appear in different accounts receivable groups on the balance sheet? Or do you just want to be able to sort them by delivery routes? The latter is very easily done with custom fields. See the guide below. Relevant information is under the heading, Editing or adding Customer information:
https://forum.manager.io/t/creating-and-using-customers/7018