Are you able to see a blue arrow in top left of your screen on Google.com homepage? If you are not able to see it then you need to login into your Google account and then you will notice a blue arrow pointing towards something in your screen.
It is pointing to the upper left hand corner, where you’ll see your first name with a little + symbol before it. After clicking on it you will be taken to Google+ page and if you haven’t already signed up yet then sign up for Google+, it will take you to sign up for Google+.
This is one of the first promotions it is doing for Google+ on Google.com homepage and it would take Google+ users number to somewhere near to Facebook. After three months the service has opened for direct registrations where earlier it was only available to register on invitations.