A meeting includes others. Click Invite Attendee to start your meeting invitation.
An appointment is just for yourself. When others are involved, create a meeting:

On the
Appointment tab, click
Invite Attendees.

A
To button and box appear. Type names directly in the box or click the
To button to add invitees by selecting from a list.

Once you've entered all of the meeting details, click
Send to send the invitation to the other meeting participants.