Hola Ricky

Sometimes "a" has the same function as "para" (for), which is the case in this sentence:

"¿Quién prepara la fiesta a Sofía?" You could also say ""¿Quién prepara la fiesta para Sofía?"

In the second sentence you mention, "Nosotros les preparamos la cena a nuestros invitados.", the indirect object "nuestros invitados" is referred to twice: with "les" (to them) and with "a" (to our guests). Sometimes we admite the double indirect object in a sentence. Here are other similar sentences:

"Le he regalado una flor a Carmen" (I bought a flower for Carmen.)

"Les compré a los niños unos abrigos nuevos." (I bought the children some new coats.)

I hope this clarifies your doubt.

Inma