I think anyone that said "approach your wife first" has got it dead on. This kind of decision really needs to be between the two of you before anyone else is involved.

Also remember, some women are scared of what you will think of them if they express a desire of some other dick (like this may be a trap) so prep this last conversation with many comments about how you are not insecure sexually and do not think she could ever love anyone but you.

Me and the missus treat a little group sex like an amusement park. Its fun for the evening but you can't live there.

Also, no matter how far it has gone that night, even if you literally have some neighbor box in your hand, if she is uncomfortable for even a moment...end it immediatly. The feelings a person gets from that do not go away easily.