Google-apps – Google Apps error: Currently this single sign-in feature is disabled for the domain example.com

Google+ (and the +1 button) require a Google Profile, and Google hasn't activated Profiles for Apps users yet.

Google stated back in March that Profiles were "coming soon" for Apps, but there's still no sign of it.

UPDATE (July 21)

Still no sign of Profiles/Plus for Apps users, but Googlers keep saying it's coming "in the coming months".

John Costigan, Technical Lead for Google Profiles:

"We're actively working on making Profiles (and Google+) available for Google Apps - it should be available in the coming months." (June 28)

Dave Girouard, President of Enterprise at Google:

"Making Google+ work for Google Apps users is a very high priority for the team. Sorry to make you wait - we have to do it right." (June 29)

That's exactly how it works and is by design. Once you set it up, all it does it make it easier for you to hand out to your users and for them to get to the login page.

Set up URL forwarding

Google Apps offers a URL forwarding feature called custom URLs. This allows you to create simple, easy-to-remember URLs for the services available in the core Google Apps suite. For instance, a custom URL allows you and your users to reach the email login page at http://mail.example.com. You'll need to create a CNAME record in your domain settings in order to set up a custom URL.

You can then use either the custom sub-domain you've set up or the longer full Google Apps URL. Either one works, but only the latter is the one that appears in your location bar.