Is the microphone switched on automatically when someone joins an xbox party?

I just noticed at a relatively bad point in time that a friend had joined my xbox list in one of my started parties. Was he now able to hear the whole process or does the microphone have to be activated first?

The microphone is immediately active, controlled by a voice activation method. To prevent this, you have to mute the microphone manually or mute it in the settings.

That's why I love my PC, there's something like push to talk, everyone only gets what they're supposed to.