If you need to reply all the meeting attendants to follow up, please see the steps below:
Step 1: Open the meeting (to pop out the meeting message in a separate window);
Step 2: In the new window, click the "Meeting" or "Meeting Occurrence" if that is a meeting with the occurrence schedule;
Step 3: Click "Respond" in the Respond section;
Step 4: Select "Reply All" from the drop-down list. From here, you will have the message window with all attendants in the "To" box.