How do I email everyone in a meeting invitation in Outlook?

Find the event in your calendar. Right click on the calendar entry and there should be a 'Reply All' option. It's greyed out in my screen capture because I don't have anyone else invited to the appointment, but it illustrates the answer...

For Outlook 2013...

If you have the meeting item opened, you can go to the Meeting menu and click Respond to find Reply All...

I found it under the Actions menu of the appointment, Reply to All with Message.

You may have to expand the Actions menu as I did in order to see this option.