Over the past 90 days or so, Google has been testing it's new Google+ social network. As of yet, sign ups for the network have been fairly limited; as you need to be invited from someone already using the service.
Throughout the 90 day period dozens of 'changes' have been made since the social network original public testing release. The big 100th change is 'Open Signups'. This should mean that now anyone can sign up for the service - with or without an invite. Personally I would like to see Google+ gain a larger audience: I much prefer it over the clutter of other social networks.