(The following is from Microsoft Outlook 2010 for Windows. Other versions should be similar.)
To schedule a meeting in Outlook:
Either click "Calendar" on the bottom left of Outlook.
When the calendar opens, click "New Meeting" on the top left of the screen.
... 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 Assistant".
The following screen will open, showing everyone's schedule, including your own. In this case, two other people are busy.
Drag the boxed-out area to a location where everyone is free, as shown below. The time will also change.
The time is changed to where everyone is available.
Click "Send" to schedule your meeting.