Open Outlook
Go to Calendar
Under My Calendars, find a calendar with your name (followed by the @ symbol). Right click on your calendar and select Properties
Go to the Permissions tab and click on the Add... button
Add the user by searching the users name. Once you find the user, select the users name and click on the Add-> button and then click OK
Select Contributor on the Permission Level: field (make sure you are applying this to the right user and that the Create items and Folder visible are checked) and click Apply
After this, the user you selected will have the ability to add calendar meetings to your calendar.