Note: Before proceeding, please review Best Practices for Sharing and Delegation – Office 365

You can choose to grant individuals access to specific mailbox items.

Outlook on Windows (2010/2013)

1. In the navigation pane, select the folder you want to share:
   

2. In the Folder tab, click Folder Permissions:
     

3. Click Add…:
     
4. Search for the person you want by name. Click the Add -> button, and press OK once you’re done adding users:
    
5. Customize their permissions for this folder by using the Permission Level drop-down menu and the checkboxes below:
   
    
6. Click OK once you’ve finished setting permissions.

Note: It is not recommended to share your entire mailbox. If you need to grant this level of access to someone, adding them as a delegate is the best option. See the following guide for Assigning Delegate Permissions

Outlook for Mac

If you use Outlook for Mac 2011, please see the following guide: Share a Folder in an Exchange Account

Outlook Web Application

1. Log in to OWA
2. Right-click the folder you wish to share
3. Click "Permissions"
4. Click the + icon at the top of the window, enter the name or email address of the user to share, select the user, and click "Add"
5. Select the appropriate permission level and click "OK"