When you need to get in contact with those attending a meeting you scheduled, there is a quick way to do this from the original meeting on your calendar.
1. Open your Outlook Calendar.
2. Open the meeting you scheduled.
3. On the "Meeting" tab click "Contact Attendees" and choose the method you would like to contact them. Note that the IM feature will require the use of Lync and the Call feature is disabled.
4. Prepare and send the message that you wish to share with the attendees. You will notice that if you have a room reserved, it is excluded from the message.
These instructions, along with illustrations and instructions for additional features of Conditional Formatting for Message Lists, can also be found in SharePoint > Software Users Group > Shared Documents > Outlook-Office 365 > Contacting Meeting Attendees.