I've gotten an invite through the real invite thing and the "tag trick", but i still get the "We've temporarily exceeded our capacity. Please try again soon" message. I'm starting to think that google just doesn't love me.
Keep trying. It can take a while. It was nearly 24 hours between when I received an email with a join link and finally getting past the "temporarily exceeded capcity" message.
Its been over 36 hours for me and I'm still not in.