How to use Office 365 shared calendars outside your organization

1. Open your Outlook calendar, under the list of calendars on the left, select New Calendar.
2. Label the calendar.
3. Right click on the calendar name and select Sharing & Permissions.
4. Add the email address of the external person you want to share with. Since you’re sharing your calendar (the room calendar) with a person outside your organization, you will not be able to give them edit permissions.
5. Select the level of permissions for shared calendar visibility.
   Can view when I'm busy allows others outside your organization to see only event times. If you’re comfortable with sharing more information, Can view titles & locations details will show Free/Busy time, titles, & location. 
6. Repeat these steps for each of your room calendars in Office 365.
   If the person you shared the calendar with uses Office 365, Outlook, or another kind of Exchange app they will receive an invitation and be able to add your calendar to their list. 
7. cross platform permission issues
   These steps on how to use Office 365 shared calendar with external users have a couple limitations for the average organization.
   The above process only works for people also on Exchange or Office 365. Google users can view the calendar via a link emailed from Outlook. However, they will not be able to access your shared calendar invitation without a 3rd party app due to different calendar formats.