Learn Spanish (Latin America): ¿Fueron ustedes a la excursión ayer? - Did you all go on the excursion yesterday?

You have to look at the context. Because fueron is followed by the preposition a (to) and an event or destination (la excursión), it clearly comes from the verb ir (to go).

Why does ustedes use fueron if that is the 'they' form?

In Latin American Spanish, the plural 'you' is always ustedes. Even though it means 'you all', it strictly shares the exact same verb conjugations as ellos and ellas (they).

Why is ustedes placed after the verb fueron?

In Spanish questions, it is very common to switch the word order and put the subject right after the verb. You could also ask ¿Ustedes fueron a la excursión ayer?, but putting the verb first sounds very natural for a question.

Why is the word a needed before la excursión?

Whenever you use a form of the verb ir (to go) to talk about moving toward a destination or event, you almost always need to follow it with the preposition a (to).