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Go to the Calendar icon on the left side of the screen:
Towards the middle of the screen locate and click the Share Calendar icon:
Under your email account click on the calendar icon:
Click the "Add"button and type in the person you would like to share your Calendar with:
Once the person is added you can change their permissions accordingly and select "Apply":
Permissions and what they do:
Can view when I'm busy - Allows others to see your availability (free/busy status) on your calendar, without any event details.
Can view titles and locations - In addition to seeing your availability, others can view the titles and locations of your meetings.
Can view all details - Grants access to full event information, including descriptions, attendees, and all details mentioned above.
Can Edit - Allows others to modify your calendar events. This includes changing times, titles, locations, and other details, along with all previous permissions.
Delegate - Provides full control over your calendar. Delegates can create, modify, and delete events on your behalf, in addition to having all the permissions listed above.