Public Folders are created through Outlook. Simply change the view to Folders. You should see All Public Folders. Create a folder under there, then adjust the permissions on the properties of the folder.
For the inbox, again just right click on the folder and choose Properties. Click on permissions and adjust.
For sub folders you have to do a little more so they can open them.
http://www.amset.info/outlook/sharing-non-default-folders.aspSimon.