Google had announced last week that they would soon launch a new feature that would allow you to chat with people in your circles without sharing your email addresses.
They have apparently started rolling out this new feature and you can set it up as per your preference.
You can opt out or pick individual circles that you want to connect with and the chat client in Plus would work accordingly.
Google had also said that this feature would also work in third party clients so that’s good news. Users are apparently being assigned customized identification id in format email@example.com