Tried with this workaround: Create a circle, add a person to it, then share some content in the circle. Everyone who isn't yet member of Google+ gets an invite. Let us know if it's still working.
When about an hour ago, someone to whom I sent an invite clicked on the big orange button in the email, they got a page telling them, "Already invited? We've temporarily exceeded our capacity. Please try again soon."
They are rate limiting new sign ups. The person doesn't need to get a new invite, they just have to try the invite again later.
thanks - haven't got an invite yet however ? will let you know if I get one :) [otherwise won't update] thanks ;)
I got an invite two days ago and had the same problem. However when I installed the Android Google+ app, it allowed me to register start using it immediately.