Learn Spanish (Latin America): ¿Bailas conmigo en la discoteca?

The preposition con (with) has a couple of irregular forms. When you want to say "with me" or "with you" (informal), you don't use "con mí" or "con ti". Instead, the words combine into the special single words conmigo (with me) and contigo (with you).

Why does bailas translate to "Will you dance" when it is just the present tense "you dance"?

In Spanish, the present tense is very flexible and is often used to make invitations or ask about immediate plans. While ¿Bailas? literally means "Do you dance?", in this setting it functions as a casual invitation meaning "Will you dance?" or "Do you want to dance?"

Why use en for "at the club"? Doesn't it mean "in"?

en is a very versatile word. While it often means "in", it is also used for "on" or "at" when talking about locations. Spanish simply uses en as its go-to word for being located somewhere, so en la discoteca perfectly translates to "at the club".