Hey Claire,
A shared calendar is usually an individual's calendar which has been shared with other users. A group calendar is commonly a single calendar stored in a Public Folder on an Exchange server.
An Exchange server is capable of sharing a calendar between users. It is also your only option to share a calendar between users if you're using Outlook for Mac 2011. A shared calendar or a group calendar will accomplish this in Outlook for Mac 2011 without any issue.
Office 365 includes Exchange. Depending on your Office 365 plan, it includes different features. You can compare the plans on the following link:
**Office 365 | Compare Plans**
Hitting the "Plan Advisor Tool" link at the top will walk you through the process of choosing the best plan for your company.
If you have further questions about Office 365, I'd recommend posting your question in the Office 365 Community by following the link below:
Hope this helps!