She seems to have a gift. She may well be seeing other people that you can't. Just speak to her openly about these 'others' and just make sure they're not frightening her (if they are, help empower her by teaching her to tell them to 'go away'). If they're not frightening her - ask her to tell you about them. You may be able to research and find out if they are passed souls!

Don't be afraid. It's quite natural and lots of kids can see/hear/smell things that we can't! We unlearn it as we get older...