On occasion I need to manage Exchange mailboxes. Generally I just do two simple tasks: Send As and Calendar permissions. The former can be easily managed via EMC, the latter cannot. While it can be managed from the sharing calendar user’s Outlook client, that is annoying and can require human interaction. Let's keep that kind of stuff to a minimum. It’s much easier to user PowerShell. Be sure to use the Exchange Powershell Module.
You can look at a user’s permissions with this cmdlet:
After entering the cmdlet you type in a user mailbox for Identity. You can also add the –Identity into the cmdlet directly to define. Either way, this will display the current permissions for that user.
To give a user to have permissions to another calendar use this cmdlet:
Add-MailboxFolderPermission -Identity TheUserCalendarYouWantToShare@somemail.com:\Calendar -User GivingRightsToThisUser@somemail.com -AccessRights Reviewer
Adjust the AccessRights as needed, ie: Owner, Editor, Author, Reviewer, etc…