I recently needed to create a .csv file showing all permissions that users had to each others mailboxes (including shared).
If you need to find out exactly who is accessing all of your organisations mailboxes use:
Get-Mailbox -RecipientTypeDetails UserMailbox,SharedMailbox -ResultSize Unlimited | Get-MailboxPermission | export-csv c:\temp\SharedMailboxUsers.csv
If you only want to look at Shared mailboxes simply remove the UserMailbox parameter.
