restrict access to users of one site to another that is on the same multisite

Hi there!… This WordPress 3.0 with the multisite feature is my salvation… thank you very much for the great job!

But i am having an issue that i can´t resolve in a simple way…

I am creating many sites with this WP multisite… But all these sites are private. So… Just the users that i have allowed can access to the site. But each site has different users allowed …
So … for example: The user called: “pepe” can only access the web site of the Company A (companya.website.com) and the user “Tony” can only access the web site of Company B (companyb.website.com)

The issue is that even in the dashboard each user only belongs to its respective website … “pepe” user can log in perfectly in the company’s web site B and see it´s content… and then company B will kill me for shareing their private content to their competence!

I tested all existing plugins that restrict access to the site only to registered users but it seems that no one has updated to work in wp 3.0 with multisite enabled.

Any solution to this problem that could be fixed in a simple way when having many users and many sites?