There is an option for person to person contact access. This isn't "sharing" of contacts between accounts like you can do with Docs but it may be a solution -and it's built into Google Apps.
It's called Delegation.
In your email settings, in the accounts tab, look for "Grant access to your account:".
Watch this video to learn more:
http://www.youtube.com/wa
You could set this up on a person-to-person basis.
Also, when Google+ becomes available as a service in google apps, you will be able to construct circles and share them with others in your domain. This may ultimately be what you are looking for.
Is there any workaround for sharing contacts (person to person) in Google Apps?
