Can I limit access to the wiki to approved viewers only?

I am using WPMU NELO and I have installed the WIKI plugin. I would like to have several WIKIs in action, each for a different working group. All members of our team should be able to see the main site and blog, but only Group A should be able to see Wiki A, and only Group B see Wiki B.

How can I do that?

Thanks in avance for your help./
Kind Regards ...