One of these solutions should solve your problem, but I would like to point out that I strongly advise against doing method 1. There are problems with this method which I'm sure others with more Knowledge of SME's workings will point out. I only used this method due to the imminent failure of an SME system and I needed a quick drag and drop backup solution, i.e. I used this on a failing, soon to be scrapped system I wouldn't recommend it on a live system as I do not fully understand the consequences.
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Method 1
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1) login to your SME box locally or across the network using 'PuTTY' or similar. Login using 'root' as your username and your normal admin password.
2) At the prompt type 'mc' and press enter to load Midnight Commander.
3) familiarise yourself with Midnight Commander if you are not already
4) Go to /etc/samba and locate the file 'smb.conf' and press F4 to edit it.
5) At the top of the file you will notice it says do not edit, so edit this at your own discretion I will NOT be responsible if this messes up your system.
6) Go to the bottom of the file and enter the following (anything in
{} are comments for your information, do not enter them or there contents).
[Share name]
{Call this whatever you like but just remember naming conventions for iBays i.e. keep it short n to the point and in lower case, I called my share 'all'}comment = Share description
{Call this whatever you like} # Recycle Bin disabled
path = /home/e-smith/files/ibays
read only = no
writable = yes
printable = no
inherit permissions = yes
create mode = 660
{This I copied from an iBay I had created properly you may need a different code} 7) Press F10 to save the file, and the F10 again to quit Midnight Commander.

You will now have a new share in Windows. But be careful with it, the
directory structure has a couple of extra folders for each iBay be careful not to delete or modify them.
9) From what I understand, any changes to iBays made in the web console will re-write this file and remove any changes you make, so this really isn't a solution for a live system.
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Method 2
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Unless you need to prevent certain users from accessing certain iBays and you are not running windows 2000 or above in a domain, I would create 1 iBay and map a drive to it. You can also map drives to folders within that iBay for other users. For example in your logon script.
net use M: "\\servername\iBay name"
This will map the iBay to the M: drive
net use P: "\\servername\iBay name\accounting"
net use S: "\\servername\iBay name\sales"
net use J: "\\servername\iBay name\janedoe"
Out of the 2 methods I mention, this is the best option on a 'live' system as long as your machines are all on a domain and are windows 2000 or above, you should have no problems locking down the sub folders in the iBay.