Share calendars with external users
It's often necessary to schedule meetings with people outside your organization. To simplify the process of finding mutually agreeable meeting times, Office 365 enables you to make calendars available to "external users," those who need to see free/busy time but don't have user accounts for your Office 365 environment.
Calendar sharing is a global setting, meaning that you, the admin, can enable it for all users in the tenant. Once sharing is enabled, users can use Outlook Web App to share their calendars with anyone inside or outside the organization. People inside the organization can view the shared calendar side-by-side with their own. People outside the organization will be sent a URL that they can use to view the calendar. Users decide when to share, how much to share, and when to keep their calendars private.
If you want to share calendars with an organization that uses Exchange Server 2013 (an on-premises solution), the Exchange administrator will need to set up an authentication relationship with the cloud. This is known as "federation" and must meet minimum software requirements. See Sharing for more information.
Enable calendar sharing using the Microsoft 365 admin center
In the admin center, go to the Settings > Services & add-ins page.
On the Services & add-ins page, select Calendar.
On the Calendar page that opens, choose whether you want to let your users share their calendars with people outside of your organization who have Office 365 or Exchange.
Choose whether you want to allow anonymous users (users without logon credentials) to access calendars via an email invitation.
Choose what type of calendar information to make available to users. You can allow all information, or limit it to time only or time, subject, and location only.
Invite people to access calendars
Once sharing is enabled for the tenant, calendar owners can extend invitations to specific users. See Sharing your calendar in Outlook Web App for instructions.