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1. On the Home tab of your calendar, select "Share Calendar" in the Manage Calendars section.
2. To add an individual , select "Add..." then use the address book to select the person or people to whom you wish to give a certain permission level. Then select the "Permission Level:" menu to set the level.  You can hover on different levels to see what the detailed permissions for each level are.
3. To change the permission level for a person or group of people, click the person/group, then use the "Permission Level:" drop-down menu to select a different level. 

1. Select "OK" if you are done making changes, or "Apply" if you want to see the changes you've just made, but still have more changes to make.