To view current permissions on a public folder:
Get-PublicFolderClientPermission -Identity “\FOLDER NAME HERE”
To view current permissions for a particular user:
Get-PublicFolderClientPermission -Identity “\FOLDER NAME HERE” -User USERNAME
To add a new permission for a user:
Add-PublicFolderClientPermission -Identity “\FOLDER NAME HERE” -User USERNAME -AccessRights PublishingEditor
You can view definitions of permissions and obtain more information at this URL: http://technet.microsoft.com/en-us/library/aa998834(EXCHG.80).aspx