If an appointment is already on your Calendar, whether scheduled by you or another user, and you need to invite additional people, you can use the Invite option.
To invite other users to appointments:
In the Calendar view, right-click the appointment and select Invite.
Begin typing a name, select it in the list, type another name, and so on until all invitees are listed.
(Optional) If needed, you can add an attachment or information in the message body.
Click Send.