Anyone got a definitive guide on how to do this correctly?  I know on the surface it looks nice and simple and plenty of guides tell you what to do but it never seems to work out that way.  

I have a customer who has 7 email accounts on Office365,  he wants all other 6 to have full access to his emails.  Calendar sharing hasnt been a problem but most of the time when I have shared out the folders it'll come up with everyone else not having permission.  

Is there an easy way to do this from the admin panel or does it need to be done on each persons account?  Ive read some guides that have said share it out via outlook (desktop), some via the online portal, some via admin.  I can never remember which is the correct and actual working way of doing it. 

Im not after a "shared" mailbox, know how to do them no problems at all.  Im after sharing a particular persons mailbox with particular people in the organisation. 

I just found this: Accessing other people's mailboxes in Office 365 so I might give that a shot although it doesnt look anything different to what Id normally do.