Here are the steps to add a domain to your postini account:
1. Go to https://login.postini.com and login to your Authorized Admin Postini Account.
2. Click on Show Hierarchy on upper left corner.
3. Click on <InsertCompanyName> Users.
4. Click on 'Domains' on the upper middle tab under 'Orgs and Users'.
5. You will see your current domains in this page. To add a new domain, click on 'Add Domain' in upper right corner of the page.
6. Enter your new domain name and any alias that you want for that domain.
7. Click on save.