Technically, "gratias tibi ago" or "gratias vobis ago" .
Vobis is plural. Tibi is singular. They both mean "you" and are in the dative case. Typically, Latin-type people formally thank each other by saying gratias tibi ago or some facsimile thereof, but there are other ways of thanking that are less formal and seem more like the modern English 'thanks' or its equivalents in Romance languages (e.g., French "merci").
However, to say 'thanks' or 'no, thanks', you can just use the adverb benigne. Whether it's an acceptance or a polite rejection depends on what you do when you say it. Benigne ades means 'Nice of you to come".Benigne dicis means 'Nice of you to say so', which is an appropriate way to accept a compliment.
