Here we got an official confirmation from Google+ team member Dave Girouard who says,
Please tell your friends using Google Apps that we’re working to make Google+ available to them. Sorry for the delay – we need to do it right!
what is Google+ ?
How to Use Google+ without an invitation?
1° – You need to have Google Chrome installed.
2° – Install the Google’s official Google+ / Google Plus Chrome app
Click on it, it will take you to Google+ site and asks you to sign in using Google account. If you have already logged in then it will allow you to join Google+.
That’s it. You are done!