Someone is sharing with me on Google+ but I can’t join. What gives?
That’s great that someone wants to share with you! And while you can check out what they wanted you to see, you can’t quite yet join Google+. Google+ is in limited Field Trial. During this time, when something is shared with you by someone on Google+, you'll receive it via email but won't be able to comment or engage with the content like other Google+ users. You'll be able to join Google+ as we let more users in over time.
In the meantime, you can poke around hereto see all the cool things that you’ll be able to do with Google+.