Instructions
Click on the avatar of the Sim throwing the party. Click on a phone in the house and click "Throw Party." Select the type of party to throw. Pick guests from the list and click the check mark to invite them.
Click on the phone again. Click on a Sim's avatar from the listing who wasn't invited to the party. Call him and invite him over.
Call as many guests as desired. Stop calling the Sims about two minutes after the party starts. If you invite someone over too late, they will miss the party.