(The following is from Microsoft Outlook 2011 for Mac. Other versions should be similar.)
To schedule a meeting in Outlook:
Either click "Calendar" on the bottom left of Outlook.
... OR click "New Items" > "Meeting" on the top left of Outlook.
Add your attendees and the time.
To check availability of your attendees, click "Scheduling" or "Scheduling Assistant" .
The bottom of the screen shows the availability of your attendees. The following screen shows that almost everyone is busy.
Drag the boxed-out area to a location where everyone is free, as shown below. The time will also change.
You can edit your message by clicking "Message" or click "Send" to schedule your meeting.