Players must first buy a mansion to throw a party in a mansion in a mansion in BitLife and complete the Mansion Party achievement.
Start by creating a character and aging up with the goal of having a decent job that pays a ton of money. The best jobs to shoot for would be an athlete, musician, and pop star job. Also, any famous job would be the best to go with as they often make millions of dollars.
That said, once you have a famous job and money, you can buy a mansion via the realtor tab under the go shopping tab in BitLife. View the listings, and you will be able to both find and purchase a mansion for a price that varies depending on the type of mansion.
Once you purchase a mansion, you will then need to view the mansion under assets, which you can find on the main menu screen next to both age and jobs.
When you find assets, click on your mansion, and there will be a vast list of options to choose.
Ignore all of them except the one labeled party and click it. Doing so will have your character host a party at your mansion, and it will complete the Mansion Party achievement.