Microsoft added the ability to the Outlook app on Android and iOS to view and edit shared calendars in 2017. It can only open a calendar to which it has been invited. The step-by-step guide starts with sending that invite.
- Browse to Outlook on the Web at https://outlook.office.com
- Sign on to the mailbox you want to share, or if you already have access to a shared mailbox, sign on to your own mailbox.
- Select Calendar from the office menu
- If you want to send the invitation to a shared calendar, open the shared calendar by clicking on your account icon and selecting "Open Other Mailbox"
- Enter the shared mailbox name or part of the name, select the mailbox when it appears in the search list, then click Open.
- Open the navigation menu at left by clicking the menu button.
- Right-click on Calendar and select Sharing and permissions
- Enter the email address to share to, even if you already have permissions, and click Share. Remember, the Outlook mobile app needs an invitation in order to open the shared calendar. Set the permission level also, even if that has already been previously set.
- Check your inbox from the Outlook mobile app and click Accept and then View to open the shared calendar in Outlook mobile.