(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.
(And check Availability)
Schedule a meeting
Create a meeting request in one of the following ways:
From the Inbox, select New Items > Meeting or use the keyboard shortcut Ctrl + Shift + Q.
From the calendar, select New Meeting or use the keyboard shortcut Ctrl + Shift + Q.
From an email message in the Inbox, select the Reply with Meeting button.
Add attendees in one of two ways:
Add your attendees to the To line of the meeting request and skip to step 4.
If you're using an Office 365 or an Exchange account, use the Scheduling Assistant to see attendee schedules.
In the Scheduling Assistant, either click the box under your name and type an attendee's name or email address or click the Add Attendees button to bring up your address book.
Click the icon next to the attendee's name in the Scheduling Assistant to mark a specific attendee as a Required Attendee ,Optional Attendee, or a Resource. When you start typing a attendee's name, Outlook will provide suggestions for you based on the people you've met with or emailed recently. Choose a name from the list to add an attendee.
Add a meeting subject, start time and end time, and location.
In Outlook 2016 and Outlook for Office 365, you can start typing a location's name, for example, Fourth Coffee Shop or Conference Room 23, and Outlook will look for location matches. Select one of the suggestions to choose that location, or continue typing to enter another location.
To attach a file to a meeting request, on the Insert menu, select Attach File, then choose the file you want to add.
If you don't want to make this a recurring meeting, click Send. Otherwise, see Make a meeting recurring.