Add the frozen meatballs, grape jelly, and chili sauce to the slow cooker and stir everything together so the meatballs are well-coated.
Set your slow cooker to low heat for 4-5 hours or high heat for 2-3 hours, depending on your time constraints. Stir every 30 minutes to keep the meatballs coated in the sauce.
Serve in the slow cooker to keep them warm through your event.