To your credit, you already know the answer - talking about it!
Key is to find some time outside of your busy day where you can be alone and have some quiet time to talk without distraction. Perhaps take her out for drinks. Perhaps just after you get into bed at night. There is no doubt that talking about it for the first time where its a serious possibility is a bit nerve racking. Most of us on this board have been there.
Talk it out. Assure her that you are comfortable. Lay down some ground rules, etc. Also, don't necessarily consider the first conversation the "decision" maker. Talk over a few days so that it begins to settle in and you can discuss any other questions, thoughts, concerns that pop up.
As to the potential playmate, some may caution about playing with friends. Hard to know what your relationship is with this particular person by your description, but something to consider. Don't want to ruin a good friendship over a lifestyle experience.
Most of all, have fun!