This document outlines various roles and permissions available in Outlook for sharing folders; calendars and items. Most often you just need to right click on the item; calendar or folder and click on Properties. After clicking Properties Select Permissions and then click on Add to add users. You can refer the table given below for access definitions.

Name of Roles. Tasks which can be performed.
Owner Create, read, modify, and delete all items and files, and create subfolders. As the folder owner, you can change the permission levels that other people have for the folder. (Does not apply to delegates.)
Publishing Editor Create, read, modify, and delete all items and files, and create subfolders. (Does not apply to delegates.)
Editor Create, read, modify, and delete all items and files.
Publishing Author Create and read items and files, create subfolders, and modify and delete items and files that you create. (Does not apply to delegates.)
Author Create and read items and files, and modify and delete items and files that you create.
Contributor Create items and files only. The contents of the folder do not appear. (Does not apply to delegates.)
Reviewer Read items and files only.
Custom Perform activities defined by the folder owner. (Does not apply to delegates.)
None You have no permission. You cannot open the folder.

You can create custom permissions by selecting the appropriate check boxes and options under Permissions.

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