It is not possible to share only few users.
The concept of the sharing of extensions is to share the MySQL tables and therefore all the records.
So this is correct that all the users are shared between the websites.
If you want to restrict the access to some articles or some content, you have to investigate the usage of ACL application.
Perhaps (not guarantee) that NoixACL could help you.
We can just tell you that this NoixACL is an extension that can also be shared.
If you are using "forms" extension, you could use CKForms that has the advantage that it can be shared. For your info, some customer requested us to modify this CKForms to restrict the access to shared records created by a slave site ID. So if you need such things, we could investigate with you if such specific extension could do what you request.
This is possible with our billable support
www.jms2win.com/download?page=shop.produ...35&category_id=1
The idea at the origin was to have specfic "leads form" that are filled in slave site and allow the manager of each slave site to see only their records but the headoffice of the company requested to see all the leads of all the dealers.