Before setting up Office 365, you need to check the version of your MacBook to see whether your Mac OS is the latest version because different version of Mac OS will have different problem when you set up your email.
How to check your Mac OS
Click the about this MAC
It will show you the version of the MAC OS
How to setup Shared Mailbox on Mac Mail
Go to the Apple Logo on the top left bar -> click System Preferences
Once you are inside the System Preferences -> click the Internet Accounts -> click the Add Other Account
On the add other account, click the Mail account to add the new account or existing account.