When in Outlook click on the Calendar tab on the bottom of the screen.
Click on new meeting.
For Meetings:
Input the email address(es) of the those you wish to invite, or click on the 'To...' button to search and add recipients from address book.
Using Address Book
To add e-mail accounts from address book, search for name of the contact you wish to add in search box.
Once the name of the desired contact is found, you can add it to the box by double-clicking or highlighting and clicking on the required or optional box.
When you have completed finding the contacts you wish to add. Press OK
Once you have selected your desired attendees, enter the remaining fields. Note: If you wish to enter more information, such as details of the meeting, free text is available in the white box below the subject line.
Note: On the right side of the screen, your current schedule is displayed, it will assist you in determining if you any conflicts with the time you wish to have the meeting.
You can attach files to the meeting invite by clicking the Insert tab and clicking on Attach File.
Click on link to get more details on including attachments.