Before, the keys to Google+ were only given to those very select few who were fortunate enough to be allowed in at launch. So naturally an invite was a precious and prized commodity. Google has stressed that Google+ is in its very early stages though whatever it was that Google was aiming for with its field trial must have been pretty darn successful. The reason I say that is because every single person that was invited to the service is now able to invite other people in as well.
But that wasn't all, the people who were invited by the initial users were able to invite friends of their own too. Even better was the fact that people writing about the decision to open up the invitation process were suddenly struck with a lot of popularity amongst readers. MG Siegler from TechCrunch wrote on his Google+ page on his own story that, "I'm not sure any TechCrunch post has gotten comments at such a fast rate."
This amount of attention seems like it must have been a bit more than Google had asked for. After nearly six hours, Google shut the invitation process down over what Senior Vice President of Engineering at Google Vic Gundotra called "insane demand". Gundotra posted on his Google+ page late on Wednesday night, "We need to do this carefully and in a controlled way." Gundotra did not specify when the invite functionality would be back up and running.
Source: PC World - Google+ Invite Process Shut Down Amid 'Insane Demand'