Your sims will then arrive at the forgotten grotto cave and the kidnapped sim will be added to your household (If you don't have 8 sims already in your household).A sim selector screen will show up showing the sim that you can kidnap, click on their portrait.To kidnap a sim click on them -> Life Tragedies -> Kidnap Sim To Cave.
You can now kidnap other sims (Teens - Elders) for a ransom, or to keep them in your household as a kidnapped sim