On the Domain names page, you can add additional domain names to your Google Apps account. You can add them as separate domains or as domain aliases for your primary domain.

To add a domain to your Google Apps account:

1. Sign in to the Google Admin console. 
2. Do one of the following:
   • In the classic Admin console, click Domain settings > Domain names.
   • In the new Admin console, click Domains.
     ^{Where is it? Which Admin console do I have?}
    
3. Click Add a domain or a domain alias.
4. To add the domain as a domain alias for your primary domain, select Add a domain alias to primary-domain-name.
   
   To add the domain as a separate domain, select Add another domain. (What's the difference between domains and domain aliases?)
   
   
5. Enter the name of the domain to add to your account.
   
   You must own the domain name, and it must not be registered as part of any other Google Apps account.
   
   
6. Click Continue and verify domain ownership.
   
   If the domain name is already registered as part of a Google Apps account, an error message appears on the page. You cannot add the domain to this account unless you remove it from the existing account.
   
   
7. Choose the method to use to verify your ownership of the domain.
   
   
8. Follow the instructions to verify your ownership of the domain.
   
   
9. Click Verify and continue to setup email delivery.
   
   
10. Update the MX records for the domain.
    
    If you want to route email addressed to the new domain differently from mail addressed to the primary domain, update the MX records for the domain to route email to the desired email server. If you are configuring a dual delivery pilot, update the MX records to route email to the Google Apps mail servers.
    
    
11. Click I have completed these steps.
    

The status of the domain verification appears next to the domain name on the Domain names page of the Google Admin console. Immediately after you complete the steps above, the status should read "Updating." When your ownership of the domain has been verified, the status is "Active." For domain aliases, it can take up to 24 hours after verification for all users to receive their alias email addresses.