To Share a Calendar for Other User to View:

1. Open Outlook -> Calendar -> Share Calendar
2. Click on To... button, type in user's Lastname, Firstname to find the user you want to share calendar with, then hit Enterto search.
3. Once found the user, click on OK to close the window.
4. Under Details option, choose the proper access rights for the user.
5. Click Send to send the user sharing invitation.

To Open a Shared Calendar:

1. Open Outlook -> Calendar -> Open Calendar -> Open Shared Calendar...
2. Click on Name... button, to find the user calendar you want to open
3. Click OK to open the user's calendar

To Grant Other User Permission to Edit Your Calendar:

1. Open Outlook -> Calendar -> Calendar Permissions
2. Click on Add button, to find the user you want to have access to your calendar
3. Change Permission Level to the appropriate one, see the image below for permission levels.
4. Click OK, the user will have the permission to access your calendar

Outlook 2010 Permission Levels: