This question comes up all the time, twice just last week in fact.
Unfortunately this is not an easy process because as you found out the individual user names are not on the permissions list unless you create another user account on the same machine and then share though those user names you wish to share with
. You have to do this with all the machines you intend to do this special secret share with.
The way it is now, you can only share with Everyone or nobody unless you create two user accounts on which ever machines you need to share with.
Tim Stitt wrote this up awhile back, and I wish there was a better way to do this but there simply is not.
When the users connect, you need to have password sharing enabled (i.e. they must put in a password to enter the shared computer (they can always save this so they need not enter it next time). You now need 2 local users on the Windows 7 computer, e.g. call them user1 and user2 with their own respective passwords. Don't make them ADMIN users.
Note that you also need to remap the drives and use these logins and passwords for the drive mapping.
Now user1 has access to everything, so on the outside folder give them complete access (or read only as you desire, including access to the special folder inside). Repeat this action for user2. Now go to the special folder that you only want user1 to have access to. right click on it and select properties/security. On the permissions tab, have a look and write down the current permission settings (this is an important reducndancy step should something you do fail later). Now look for a tickbox to inherit parent permissions and untick this. Now give permissions only to those users that you want to access this folder. Be aware that you probably need SYSTEM and CREATOR OWNER on there.
Like all good plans, switch user to user1 on the Windows 7 box and test that they can locally access what they are supposed to. Repeat with User2. Now remap the drives with the new passwords and give the passwords out to the users.
This should fix your issue.
Original post> Windows 7 to Windows XP Permissions Help