Yes, both of those are correct.
Someone tell us if we can say "¿Les interesa ir a la fiesta a ustedes?"
The verb is interesarse. Here is how it's conjugated:
interesarse = (mostrar interés por algo) = take an interest in OR be interested in
Siempre me intereso por todo lo relacionado con la moda. = I always take an interest in everything related to fashion.
This verbs is similar to other verbs such as gustar. It's helpful to check out some sources online for how those work.
When there is an indirect object you need les, even when you add a ustedes to clarify who you are referring to.
I used "to go" instead of "in going" but got rejected. That's OK in English where I live although maybe a bit idiomatic.
I also did the same thing, but was rejected. Considering Ir can both mean "to go" or "going".
Why doesn't duolingo accept uds. for ustedes in the English to Spanish translation question?
For "Are you interested in going to the party?" you have a few options:
¿Te interesa a ti ir a la fiesta? ¿A ti te interesa ir a la fiesta? ¿Te interesa ir a la fiesta?
And a similar construction for "Is he interested in going to the party?":
¿Le interesa a él ir a la fiesta? ¿A él le interesa ir a la fiesta? ¿Le interesa ir a la fiesta?
That last translation is vague about "him", so we would have to know that "le" referred to "him" from context.