1) Under the calendar tab in Outlook navigate to the calendar you wish to share and right click. From the drop down select “Properties” as shown below
2) After you have select the “Properties tab, Select the “Permissions” tab from the top as shown below. The click the “add” button
3) Once you select the “Add” button, you will see the below screen. Select the users you wish to grant permission to; you can select multiple at once
4) Once you users have been added, you can change their permission level to match the need. Simply highlight the user by clicking on them (as can be seen below for Liam) and use the drop down. Owner permission will mean the user can add/remove/edit appointments in the calendar