When you right click on a Public Folder you can set the access rights.
You will see that you can set the access rights for a user, multiple users (using ctrl to select multiple ones) or to a group.
Then when you have selected the users or groups you may set the following access rights:
Here we have Howard with Read/Create/Edit/Delete access to the contact folder and the other users can read the data. This is great for when users need to view the data, but you don't want them to make any changes to it.