Learn Spanish (Latin America): Estaban atendiendo en esa ventanilla.

It looks like it, but atender is a false friend. In contexts like shops, banks, or offices, it means to serve, to help, or to wait on customers. If you want to say 'to attend' an event or a class, you would use the verb asistir.

Why use ventanilla instead of ventana for 'window'?

Ventana is a standard window you look out of in a house or building. A ventanilla (literally a little window) refers specifically to a service window, ticket counter, or bank teller window where customers are helped.

Why use estaban atendiendo instead of just the past tense atendieron?

Estaban atendiendo is the past continuous tense, formed with the imperfect of estar and the gerund atendiendo. It emphasizes that the action was ongoing or in progress at a specific moment in the past, just like 'were serving' in English. If you used the simple past atendieron, it would sound like a single completed event.

Why isn't the word for 'they' included in the Spanish sentence?

In Spanish, the subject pronoun is often dropped because the verb ending already tells you who is doing the action. The ending of estaban tells us the subject is 'they' or 'you all'. You do not need to add ellos or ellas unless you want to emphasize it or avoid confusion.