File sharing between macs

I have set up file sharing on imac computer, but when people connect to the computer are you able to control the folder people have access to. I have 2 account on one computer, and when you link to the computer, one account links you to the root of that account, the other links to the drop box. How is this controled? any help i would be very grateful.
Nic

When they log in as guest they only have access to your drop box. If they log in with a username and password, they have access to anything that user does.