To throw a party click on your Sim > Cell phone > and select throw a party. Pick eight Sims to attend, try choosing Sims you are most friendly with because this will help. Set a time preferably one hour from the present time, so you have time to prepare the party. Buy equipment that will help the Sims have a good time like a buffet, drinks bar, balloons, and a stereo system. I hosted the party at the back of my house with all of these items in the garden. After the guests arrive greet them and be friendly to them. Make sure you greet each one at least once.
Buying "Legendary Host" from your Lifetime Rewards, which costs 5,000 points, will improve your guests response to the party. When a few of your guests are arrive at the party use the "Good Times" Karma power (30 Points) which may help improve their response to the party even more. The party will only last a couple of hours and you can save before the party starts, in case you mess up, to make it easier.