When you created the customer in admin-panel-Customers/Add, you can choose from Settings: Can create subaccounts option
promoting the customer to the reseller.
Bellow this option, the bunch options are shown regarding its subaccounts (user rights, order pages, products, invoices... )
Also, the existing customer can be promoted to the reseller, the same option should be ticked (in the Admin Panel - Misc).
Reseller's setting could be set in Admin Panel - Settings - Reseller configuration