"Are you interested in going to the party?"
Translation:¿Les interesa a ustedes ir a la fiesta?
Duolingo already has a lesson which covers the use of the Indirect Object usually with the 3 verbs gustar/interesar/encantar (there are thirteen others). It is worthwhile reading all of the contents of the following link but briefly. Use the Indirect Object (me,te,le,nos,les) with interesar and 15 other similar verbs. Where there is ambiguity (eg le= to you formal/to him/to her/to it) you should clarify as follows. ?Le interesa a usted ir a la fiesta? = Are you (third person singular formal) interested in going to the party? (This solution was accepted by Duo.)
Se interesa = He/She is interested in himself/herself or You are interested in yourself
se used alone is a pronominal pronoun. It means that the subject and the object are the same.