It's an exclusivity play, to pump up demand. People like things that make them feel exclusive, so for launch someone at Google tried to increase demand by maki

It's an exclusivity play, to pump up demand. People like things that make them feel exclusive, so for launch someone at Google tried to increase demand by making G+ more exclusive with invite-only. Unfortunately this increased the barrier to entry at launch too, which for social networks whose value is tied to users generating media is a cardinal sin.