The Microsoft Garage released a Outlook addin for Office 365 subscribers, called FindTime. This addin lets recipients vote on the best time for a meeting.
To use, create an email message and click New Meeting Poll. This will open a panel similar to the room finder, displaying your available times. Select several time periods you would like to meet.
Warning: don't go overboard when you select meeting times - a tentative meeting is added to your calendar for each time period. After a meeting is scheduled, the tentative meetings are removed from the calendar.
The bar next to each time period displays the availability of the attendees in each time period. Click on the person icon ()on the right to display a list of attendees.
Click on the calendar icon to display your nearby appointments.
When you complete the steps, a link is added to the message.
The recipients receive an email with a link to the FindTime calendar, where they will pick one of your suggested times or request another time. When someone votes or requests another time, the organizer receives an email update. After everyone has voted the organizer accepts one of the times. The organizer can force a scheduled meeting at any time
A list of all meetings invitations sent through FindTime is available in your FindTime dashboard (at findtime.microsoft.com).
While you do need to use Office 365 Exchange Online to organize a meeting using FindTime, the recipients don't need use Office 365 (or have FindTime installed) to vote - it works with any email address. (Outlook.com accounts that were migrated to the Office 365 server can't send meetings using FindTime but they can vote.)
To learn more about FindTime or to install it, see FindTime.