Step 1: Open Microsoft Outlook, click on the 'Calendar' icon on the left hand side near the bottom, then select 'All Folders'
Step 2: This will show all of the calendars you own or have access to. Right click on the Calendar you wish to edit, then select 'Properties'
Step 3:In 'Properties', select the 'Permissions' tab, and click on the person whose permissions you need to change. Select what type of access they will have (owner, editor, reviewer, etc.). You can edit users' permissions at any time as necessary
Step 4: Once you have given the appropriate permissions, press 'OK' button in the Calendar Properties box