This tutorial explains usage in both Outlook Desktop and Outlook Web App.
Meeting time coordination
To pick the right meeting time, open the meeting/event window. Click on Pick a Time as shown below.
If using Outlook Web App, find the FreeBusy add-in on the upper toolbar.
The add-in pane will then load. There are two tabs, Add FreeBusy to Event and Find Time, click on the second one.
As you add or remove attendees, FreeBusy will detect changes and update the available times accordingly.
On the top of the add-in page, you have the option to select the week and the duration of the meeting.
Click on the timeslot that you wish to select. The meeting start and end times will be automatically updated.