Google+ project is Google's new product which is expecting to be a competitor for twitter and Facebook. I got an account from one of my friend. It seems that now Invitations are stopped.
If you have a Google Plus account you can invite your friends by adding them to a circle and just share one thread to that circle (or you can share a thread directly with that person). They will receive an invitation to their mail.
Google+ enabled invitation link again
Check invite link at the bottom of the right column
Add the email addresses and on the next screen add them to circles (if you already not added them to circles). That’s all.
if neither you nor your friends have an invitation of Google plus,
create a google public profile (http://profiles.google.com) and contact us using the contact us form here, we will add you to our circle (Remember you will have to wait till Google open the invitation )
Read the comment section too. If you are willing to share invitation you can comment, just like victor did. Thanks Victor.
You can request an invitation directly from google
You may share this post on twitter or Facebook so that your friends can also get one.