Okay, so the way this could have happened, by your account, is Michael Sabia is talking to you about your schedule, relaying the information back to McKinsey and then inviting McKinsey to join a call with you. Throughout this whole time, you have no idea that McKinsey is present in the meeting, while Michael Sabia is relaying information between you and McKinsey about specific details of your availability and schedule.
You want us to believe that at no point was someone from McKinsey talking directly to you or your executive assistant. It was just a complete surprise to you that McKinsey was on the call.