I’ve experienced the same trouble with my brand spanking new MBW and so I can only lend you the benefit of this particular newbie’s limited knowledge thus far.
You can change the read/write permissions by logging onto the MBW via the browser interface (depends what IP your router has assigned it but just for example with my Netgear router I put in the address http://192.168.0.5). Once there, if you click on the Advanced tab, follwed by Users and Folder Share Permissions, you can change the permissions from “public” to then grant other users “Full Access”
However this does not solve the problem of being able to access any sub-folders that you create and so far, the only means I’ve discovered of being able to access sub-folders created by another user is to go back to Finder, click on “Go” and then “Connect to server” and enter the address of your MBW as smb://… So for example mine is still called MyBookWorld and the sub-folders I want access to are in a folder I created called Various, thus the address I put in is smb://MyBookWorld/Various my laptop requests my user name and password and then when I connect I can access everything in Various
I’d be delighted if anyone here can advise all of us of a simpler/quicker method as one of the main reasons for getting the MBW was so that both my wife and I could store all our music, photos, movies etc on the MBW and both have access and keep plenty of spare memory on our laptops, but if she has to go through this palaver everytime she wants to store/access I file, I can’t honestly see her being bothered with it
Meanwhile I sincerely hope this is of some help