Normally Zen Brands are set globally or per space. But you can configure Zen Foundation to override the normal brand based on the logged in user's group membership.
To do this:
Add one line per group to the file of the form group-name=brand. For example:
would cause users who are members of the customers group to see your corporate brand, while users in the employees group would see the intranet brand. Normal global and space-level brand settings would continue to determine the brand seen by users that have neither group membership.
You don't need to restart Confluence after creating or modifying this file. However, depending on the browser, users might need to clear their browser cache to see the change take effect.