Inviting your friends on Google+ is now even easier than before. Google has made a significant change allowing user to share a single link (per account), which will direct the user who clicks on it to the Google+ sign up page. The old method of sending users emails is still there but I'm sure you will agree that this is much easier.
At the moment users are capped to 150 sign ups - meaning that a total of 150 people can sign up using your link. However I'm quite sure that Google will increase this further in the development stage. Also as long as this does not get shut down or capped (like invites for the service have been previously), Google+ will see even larger growth than present. Just think, if each user invites 150 people, then they also invite 150 people and they also invite 150 people and so on, the growth rate will be massive (you do the maths). Of course this the ideal, but I'm sure that everyone who wants to get on Google+ at this current time will be able too.
Still not on Google+? Sign up using my link here! - Free Google+ Invite