One of Outlook's most powerful features is the ability to schedule meetings with multiple people. We'll walk through the steps to make a basic appointment.
Open Outlook to your calendar. From the "Actions" menu select "New Meeting Request". This window will appear:

Click the "To..." button in the upper, left-hand area. A window will appear that looks like this:

Find the users you want to invite to your meeting just like you would when using the Global Address Book. There are three types of attendees that you can choose:
Required - Any scheduling conflict with these people will result in a failed meeting request.
Optional - A best attempt is made to schedule this person, but a conflict will not cause the meeting request to fail.
Resource - Rooms and other "resources" can be scheduled as well. They are considered "required".
When you have added all the people to the appropriate attendee type, click OK.
Set the meeting time and location and fill out any additional information you would like to include.
Click on the "Scheduling" tab in the upper, left-hand area. Something similar to this will appear:

The blue bar across the top indicates the collective "busy" time for all the people included as attendees. Find a time that is free and click at the top of that time's column. You can adjust the exact times using the "Meeting start time:" and "Meeting end time:" fields at the bottom of the window.
Now that you're done setting all the meeting information, click the
button in the upper, left-hand area to send the meeting request.