What Do The Outlook Permission Levels Mean?
If your administrator has set up a public folder or SharePoint site, you may have permission to use some or all of the folders within the public folder or SharePoint site. The extent of the activities you can perform in a public folder, shared private folder, folder you are a delegate for, or SharePoint site folder, depends on your role (or combination of permissions) in that folder, as described in the following table.
Note: Users with permission levels starting with Contributor and higher can only create Meeting Requests in the shared calendar. If you want users to create Appointments you need to set default permissions for your Calendar as Editor.