Before heading to the party, you need to pick up some appropriate attire. Drive to the marked clothing store on your mini-map. Enter the store and walk into the pink icon to change into the 'Soiree' outfit. This outfit is required to enter Cortez's party. Failing to wear it will prevent you from progressing the mission. The game will prompt you if you try to proceed without the correct clothing.
This introduces the concept of changing outfits, which can sometimes be used to reduce your wanted level or access certain areas. For now, it's purely for story progression. Once you've changed, exit the store and get back into your car.