See this link for more help:
The link David gives has a perfect explanation. To make it more understandable I look at it this way. The shared folder is the kitchen. File>open gives them access to front room only, not the kitchen. In order to see the shared folder (kitchen) they have to add the whole house (user mailbox) to their folder list, but they can’t do that unles you give them the key to the front door by granting read permissions to the mailbox itself first. Without a key to the front door, they can’t get to the kitchen. Once you grant them permission to the mailbox then when they add that mailbox to their folder list they will only see the folders you granted permission to on their folder list. Great screen shots and details are on the page for which David gave the link.