Am I missing a comma in the sentence below?
You're coming to the party aren't you?
Yes, you're a comma short! You need to add a comma just before your tag question. What's a tag question? It's a question that is added at the end of a sentence (like aren't you? in your example). It consists of two basic elements:
Many tag questions also include the adverb not, usually attached to the verb in a contracted form (n't). Here are some examples of sentences with tag questions.
So I'm lucky, am I?
Abshir isn't here, is he?
The chocolate frosting is delicious, don't you think?
The meeting is at 9:30, isn't it?
Anne will join us, won't she?
Now, back to your sentence. The tag question at the end of your sentence is aren't you. It consists of the verb are, the contraction n't and the pronoun you. You therefore need to punctuate your sentence as follows:
You're coming to the party, aren't you?