====== Allow all users to see rooms calendar ======
<code powershell>set-MailboxFolderPermission "room01:\Calendar" -user Default -AccessRights Reviewer</code>

All options:
<code powershell>Add-MailboxFolderPermission "room01:\Calendar" –User Default –AccessRights <Editor,Owner,PublishingEditor,PublishingAuthor,Author,NonEditingAuthor,Reviewer,Contributor></code>