There's nothing more maddening than trying to set up a meeting with a bunch of people and not knowing their availability. In Office 365, the free/busy information in Exchange Online saves the day. This functionality lets you know whether the person is free, busy, working elsewhere, out of the office, or has a tentative meeting on the calendar. It works not just for people but also for resources, such as conference rooms.
To see a person's free/busy information when you create a meeting and use the Scheduling Assistant, create a meeting invitation in Outlook then add the people you want to invite to the meeting. From the Ribbon, click on 'Scheduling Assistant'. If the time you picked looks blocked for any of your invitees, find a time that works best for all, then click the Send button.
