(If you have not already done so, you will need to enable Google+ for your Google Apps Domain.)
1) Open Google Chrome, navigate, and login to:
2)Click “Google Apps”
3) Click on the Google “Talk” Service
4) GoTo Google Talk Advanced Settings
5) Click the “Enable the new Hangouts for users in this organization”
(you will be prompted to review and accept the considerations in enabling Google Hangouts for your domain.)
You now have Google Hangouts enabled for your Google Apps Domain!
(Also see howto enable Google+ for your Google Apps Domain.)
Google Apps +Hangouts Domain Considerations
Domain warnings and restrictions – Hangouts does not support warnings outside of your domain and does not support restricting chat with users outside of your domain.
Chat history – Administrators can no longer disable chat history for all users. Chat history is stored automatically in Gmail for each chat unless all participants agree to disable it.
Contact sharing – Hangouts does not autocomplete from the Global Address List.
Google+ – Some features, such as multi-person video calls and photo sharing in Hangouts, require a Google+ profile.
Google Apps Vault – Hangouts is not compatible with Google Apps Vault, and you should not enable Hangouts at this time if you intend to purchase Vault.
Support – While in preview, the new Hangouts is not covered according to the terms outlined in theTechnical Support Service Guidelines.
Reverting to Talk – You can revert your users back to the Google Talk version of chat within their Gmail by disabling the new Hangouts for this organizational unit (from within Talk settings). Note: Conversations and history created while using Hangouts will still remain after reverting.