If you try and add them into the permissions your get the error :
One of more users cannot be added to the folder access list. Non-local users cannot be given rights on this server
The reason why your getting this if the user your trying to add is most likely set to a shared mailbox.
You need to convert it back to a user mailbox for it to work.
To do this drop into powershell and run the command
Set-Mailbox "mailbox display name" -Type:Regular