Everyone has a spooky tale to tell about some haunted place they've been or heard of. Ask guests to come prepared with their scariest stories and take turns with the tales. To encourage participation, offer a prize for the scariest story.
Play soft scary sound effects in the background (wind or a turbulent storm is good), and keep the lights low. Make sure to give focus to one storyteller at a time and keep hecklers at bay, or the guest may not want to divulge the scariest details.
If a participant is not good at telling stories ask them questions about the event, making sure to emphasize the emotions they felt at the time. Some people may not remember too many specific details, but they will recall the impact of a frightful event.