Is there a way to attach a group of companies to a parent company when uploading a CSV file in Sell?
It's not possible to upload or update the Parent Company field through a CSV file. The file can't be updated through the importer tool.
As a workaround, you can either update the field manually or use the Sell API to manage your contacts.
For more information, see this article: Creating a CSV file to import leads, contacts, or deals.