Adding Global Contacts for an Office 365 Tenant

You can use a public folder to store and share global email contacts, but a better approach is to use Exchange mail contacts. These objects show up in the Exchange GAL and OAB and are available to all Outlook clients (and some third-party clients too).

How to Make Public Folder Scalability Better Within Exchange Online

A recent update for Exchange Online gives extra control over who can access public folders. The change is intended to help with scalability, but it can be used to turn public folders off for any mailbox for which you care to disable access.