Many organizations own multiple Internet domains. The domains might represent distinct business units, brands, or geographies. For example, Google owns the domain names google.com, adwords.com, youtube.com, google.co.uk, and google.de (among many others). Similarly, a state university system might have separate .edu domains for each campus. Users with accounts on any of these domains need access to the same Google Apps, and they need to be able to work with users in other domains that are part of the larger organization.

You can associate multiple Internet domains with your Google Apps account, as long as you  own the domain names. Each domain remains an independent entity, but Google Apps recognizes that the domains are part of a single organization. For example, each domain has its own set of user accounts, but users can share calendars or documents with users in any domain that is part of the organization. Gmail treats mail between users in different domains as if it were intradomain traffic. You manage all of the domains using the same Google Admin console.

When you add an additional domain to your account, you can add it as a separate domain or as a domain alias for the primary domain. You can add up to 600 domains to a Google Apps account. You must own the chosen domain name, regardless of whether it is being used as a separate domain or a domain alias. Google Apps will require you to verify that you own the domain. 

Learn more about:

• What is a domain?
• Choose your primary domain
• Add a domain
• Use a domain or a domain alias?
• Add users to domains
• Move users between domains