While Free/Busy viewing access to calendars is the default setting for Dartmouth O365 services, this feature is actually only available using the Outlook for the Web (BWA) interface or Outlook for Windows client, and by design is not present for Outlook for Macintosh clients.
The minimum permission needed to open a calendar using an Outlook for Macintosh client is Reviewer, this level allows the Reviewer to see details of meetings and appointments.
As a workaround, Macintosh users can use the Outlook for the Web (BWA) interface, or from the Outlook for Macintosh client, use the scheduling assistant to view free/busy times on other calendars.