I had to do this for Resource Items, to allow all staff to view the Calendar of the resource so they could schedule items accordingly. As one of the comments mentions from this Spiceworks post…
Set-MailboxFolderPermission "roomname:\Calendar" -User Default -AccessRights LimitedDetails
This can however be expanded on knowing powershell being object oriented, roomname is actually any mailbox, :\ is the delimiter, Calendar is the SubFolder. -User $Group or $User (Groups are usually best practice per IDGLA) and finally -AccessRights, you can specify any of the access rights you see in the pull down option of Outlook.
That’s it. that’s all there is to mangling Mailbox sub-item permissions.