Folder Permissions Question

This should be an easy one ... but doesn't seem to be working the way I want it to.

I need to create a folderwhere one set of users has full control over all the folder contents and another set of users can save files into the folder but that is it - no access to change that file once it is saved, no access to view the contents of the folder, etc.

UserGroupA - Full Control
UserGroupB - ?

Anyone offer me any suggestions?

The only way I have had it working so far is:

UserGroupA - Full Control
UserGroupB - List Folder Contents
CreaterOwner - Write

However, now users can see the other files in the folder (although they are unable to view them) and they are able to modify their own file after it has been created.

Thanks in advance.

Matthew Ellis