I have an outsourced company doing our billing soon. Is it possible to add an external user to a Shared Mailbox that is under our main account. I would like to give them read/write access to only this particular address. For example, the email "firstname.lastname@example.org" should be able to view our mailbox "email@example.com". I did not see that option when I went into the Shared Mailbox section of the admin panel. Hope that is clear. Thanks.