Near as I can tell, you have to create a user, then create a Web ID based on the user, and then assign permissions to the user for just the folder you want them to have access to.
Set up user:
Create new user for client
Users tab > Users > Add new user
Set up Web ID:
Create one Web ID for each user/client
Home > Account Privileges > Grant Privileges to a new Web ID
Assign permissions to allow user into their domain(s) only
Hope this helps...