See: https://forum.emclient.com/t/exchange-office-365/55565

"unfortunately eM Client does not support Shared Calendars for Exchange / Office365 accounts, eM Client only supports calendar delegation using the EWS protocol which allows you to set permissions to your mail or calendar folders and delegate the calendar to other users. You should be able to setup the delegation in your account settings and under your account properties. Right click your account in the left pane and select Properties > Delegation to subscribe to delegated folders."