To clarify we store all of the users for your sites in the same location so everyone that has permissions to access Users section in the backend of the Sitefinity will see all of the users. Still you can setup different permissions per site for these users. You can achieve this functionality by following this steps:
1. 1) Open the selector in the upper-left corner of the page and click Manage sites.
2. 2) Click the Actions link of the site that you want to set permission for.
3. 3) In the dropdown menu, click Set permissions.
4. 4) Click Break inheritance
5. 5) Click Change button for the permissions that you want to modify
6. 6) Click Select roles or users
7. 7) Go to Users tab and select different users per site.
Do you want to have your say in the Sitefinity development roadmap? Do you want to know when a feature you requested is added or when a bug fixed? Explore the Telerik Public Issue Tracking
system and vote to affect the priority of the items