I guess I can start this off, there's a lot of info on this topic in this forum.
It all will start and finish with open communication.
Ask yourself some questions first.
Am I ready to see her with another man?
Is this just something temporary that's getting my attention, or am I ready to accept this as our lifestyle?
Is it a lifestyle or just a random fantasy that's temporarily turning you on?
How many, if any conversations have led to her mentioning interest in extra?
Has she shown interest in any kind of way?