All depends the dependencies between your websites.
If you slave site is a children of your master and that physically you store the users in the master, they will be automatically added into you master and your users will also have access to your master.
When you share the users, you have to see where the users are physically stored.
On the website where they are physically stored, all the users have access.
Under Joomla 2.5, the only thing that you can do to restrict the access is to use the "group level".
Concerning the slave sites that have a "link" to the physical website, there you have the "multisite user/site" managment tool that allows you decide on which website has the access. This tool can only be used from the websites where physically the information is stored.
So check with the JMS Tool menu / table tabs, where the user information is stored.
Put your cursor over the link icon and it will display where the link is pointing.
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