1) Start by logging into your Office 365 email account here
2) Then click on the 
in the top left of your email homepage, and choose 'Calendar'
3) On the left you will see a list of 'Your Calendars'. Right-click on the one you want to share then choose 'Share Calendar'
4) Type in the email addresses of the users you want to share the calendar with.
5) To choose what permissions they have click the drop down beside their address.
- Availability only: shows the time of items on your calendar and no other details.
- Limited details: show the time, subject, and location, but no other information.
- Full details: show the time, subject, location, and other details of all items in your calendar.
- Editor: can edit your calendar.
- Delegate: can edit your calendar, and can send and respond to meeting requests on your behalf.
6) Next, you want to hit the 'Send' button up top. This will send an invite that the recipient must accept before they can view it.
7) Last the recipient accepts the invite and they now have access to your calendar.
