access multisite

Is it possible to have granular access for multisite installations.

Consider the following setup:

A portal site called, for example - mypets.com

with the following subdomains(multisites) - cats.mypets.com, dogs.mypets.com, turtles.mypets.com

Assuming all users are registered with mypets.com, would it possible for admins to then grant access to subdomains?