In this example abaldwin is granted reviewer access to jjohnson’s calendar:
get-mailbox -identity "jjohnson"| foreach {Add-MailboxfolderPermission $_":\calendar" -User "abaldwin" -AccessRights Reviewer}
To do this for an entire Co. use Excel and a list of all users then use Concatenate to edit the individual cmdlets which can be pasted in bulk to PowerShell.