Google Apps rolled out "user policy management," which allow Premier and Education Edition customers to customize which users in their domains have access to each application in the suite. "With this feature, Google Apps admins can now start by enabling Google Wave for a group of advance testers. For e.g., at a K-12 school, admins can now enable the service for just teachers and older students, or a large enterprise can turn it on for a specific project group first.
If you're already a Google Apps Premier or Education Edition admin, you'll start seeing the 'Organizations & users' tab in the control panel over the next week," said Google.
More Info: Google Wave for your domain