Learn Spanish (Spain): ¿A ustedes les gusta viajar a España?

In Spain, ustedes is the formal plural word for "you". You would use this when speaking to a group of people you don't know well, elders, or in a professional setting. If you were asking a group of friends, you would use the informal vosotros instead.

Why do we need both A ustedes and les in the same sentence?

Verbs like gustar (to like) work backwards compared to English. The pronoun les is required here and means "to you (formal plural)". Adding A ustedes at the beginning clarifies exactly who les refers to, since les can also mean "to them". It also makes the sentence sound more polite and complete. Literally, the sentence is asking "To you all, is it pleasing to travel to Spain?"

Why is it gusta instead of gustan when we are asking multiple people?

The verb gustar matches the thing being liked, not the people who like it. Here, the thing they like is an action: viajar (to travel). When you like doing an action (a verb in its basic, dictionary form), you always use the singular form gusta no matter how many people are doing the liking.

Why do we say viajar a España instead of viajar en España?