Instructions
Open the house with the child or teen Sim who wants to throw the slumber party. Click on the face icon of the Sim who's throwing the party.
Click on the Sim's inventory icon at the bottom of the screen. Click on the phone icon to make a call. A menu of the Sim's friends appears on the screen.
Choose one of your same-aged Sim friends and call them up. Talk with them and invite them over to your house. Call as many of your friends as you want in your party.
Click on your Sim friends as they come over and click on the "friend Interaction" option from the menu and then "ask to sleep over." Do this with each of the friends at your house.