How does access control work on Mac OS X
This set up option will Share your music with others on your local network.

Settings for iTunes sharing



If we are talking about standard Folers (not iTunes) but just any folder in Finder.

Right click on the folder

Select 


  Get info


In that window look all the way at the bottom, there is 


  small lock




Open it (with your password)

Now you can modify "others" as they show up in your list the access to this folder to 


  read only


.