now google+ is open for everyone you just need a gmail id when you log in your gmail account see on your left hand you can find your name with + when you clicking on that you are going to google+ page
Firstly go to google.com in a corner u will google+ tab click there.Secondly u have to be invited by a referrer.Thirdly google will display a form of date of birth ther u have to put ur DOB then other information like first name , last name, email address, gender etc have to be filled then ur account will be created.
Its so simple just visit the Google + site then make an account with your existence Gmail account and start working there and enjoy the new features of Google +.
That's how I logged in, I seen it in the news and just ignored it and one day it was there in gmail and I clicked it and went with it.
Thanks to #2 . But it does not need to wait for the Google's Invite . We can register it now ourself .