Your saying that in the console the users can see all of the computers in their group, or every install?
If you sent them an invite to setup Hamachi:
This would be your permissions you included them in when you sent the invite. They are granted use of the entire group. If you have not created them yet, create one for each company you have, then move those computers into where they belong.
Test this and you should see the list got shorter that they can see. Now limit them to only the computer they need access to.