Adding a person

I have a question? My husband has stated that he would like to add an extra person to our fantasies! I was totally shocked! He stated he doesn't care if it is one person or a couple! I have never done anything like this do I need your help on telling me how when and how to get started and any advice you could give on completing his fantasy! I have never been with a female before so I don't know what or how to do what I need to do!