用户 (yònghù) is the standard word for 'user' in the context of technology, platforms, or services (e.g., software users, account holders). 顾客 (gùkè) and 客人 (kèrén) refer to customers/guests in a physical shop or hospitality setting; 客户 (kèhù) is a business client; 消费者 (xiāofèizhě) is the general economic 'consumer'. Use 用户 when someone is interacting with a product or system, not making a purchase.