1) In Preferences > Admin Groups, create a new admin group. Select "Not set" for all of the permissions.
2) For each user that should be able to view/edit information for specific sites only, go to Preferences > Admin Users > [Specific User] > "Admin Group Membership" tab, and assign the user to the group you created in step 1.
At this point, as long as the user is not assigned to any user groups with permissions set to "Allowed", they will not be able to do anything in the admin area. In the next steps, you will give them permission to view/edit a specific site.
3) Go to Site > User Management > Admin Groups. Click the pencil icon next to the group you created in step 1.
4) Set permissions to "Allowed" for any actions the user should be able to perform for this specific site.
5) Repeat steps 3 and 4 for any other sites that users in that admin group should be able to access.